diff --git a/.travis.yml b/.travis.yml
index c5ffd833..094635f5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,6 +5,7 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
   - "pypy"
 
 sudo: false
@@ -16,7 +17,14 @@ cache:
 env:
   - USE_OPTIONAL=true
   - USE_OPTIONAL=false
-  - SIX_VERSION=1.9 USE_OPTIONAL=true
+  - DEP_VERSION=min USE_OPTIONAL=true
+
+matrix:
+  include:
+    - python: "2.6"
+      env: PIP_VERSION=6.1.0 DEP_VERSION=min USE_OPTIONAL=true
+    - python: "3.3"
+      env: PIP_VERSION=6.1.0 DEP_VERSION=min USE_OPTIONAL=true
 
 install:
   - ./requirements-install.sh
diff --git a/requirements-install.sh b/requirements-install.sh
index 7ba9396f..59339db0 100755
--- a/requirements-install.sh
+++ b/requirements-install.sh
@@ -1,6 +1,14 @@
 #!/bin/bash -ex
 
-pip install pip==6.1.0
+if [[ $PIP_VERSION ]]; then
+  pip install "pip==$PIP_VERSION"
+else
+  pip install -U pip
+fi
+
+if [[ $DEP_VERSION == "min" ]]; then
+  sed -i'' -e 's/>=/==/g' requirements*.txt
+fi
 
 pip install -U -r requirements-test.txt
 
@@ -8,10 +16,6 @@ if [[ $USE_OPTIONAL == "true" ]]; then
   pip install -U -r requirements-optional.txt
 fi
 
-if [[ $SIX_VERSION ]]; then
-  pip install six==$SIX_VERSION
-fi
-
 if [[ $CI == "true" ]]; then
-  pip install -U codecov
+  pip install codecov
 fi
diff --git a/requirements-optional.txt b/requirements-optional.txt
index c00fd242..518360b5 100644
--- a/requirements-optional.txt
+++ b/requirements-optional.txt
@@ -2,16 +2,16 @@
 
 # We support a Genshi treewalker that can be used to serialize Genshi
 # streams.
-genshi
+genshi>=0.7,<2.0
 
 # chardet can be used as a fallback in case we are unable to determine
 # the encoding of a document.
-chardet>=2.2
+chardet>=2.2.1,<3.0
 
 # lxml is supported with its own treebuilder ("lxml") and otherwise
 # uses the standard ElementTree support
-lxml ; platform_python_implementation == 'CPython'
+lxml>=2.3,<4.0 ; platform_python_implementation == 'CPython'
 
 # DATrie can be used in place of our Python trie implementation for
 # slightly better parsing performance.
-datrie ; platform_python_implementation == 'CPython'
+datrie>=0.5,<2.0 ; platform_python_implementation == 'CPython'
diff --git a/requirements-test.txt b/requirements-test.txt
index 40df78d4..851e41b4 100644
--- a/requirements-test.txt
+++ b/requirements-test.txt
@@ -1,6 +1,8 @@
 -r requirements.txt
 
-flake8<3.0
-pytest
+flake8==2.6.2
+pycodestyle==2.0.0
+pyflakes==1.2.3
+pytest>=2.7.3,<4.0
 pytest-expect>=1.1,<2.0
-mock
+mock>=0.7,<3.0
diff --git a/requirements.txt b/requirements.txt
index 3884556f..ce36b1d0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,3 @@
-six>=1.9
-webencodings
-ordereddict ; python_version < '2.7'
+six>=1.9,<2.0
+webencodings>=0.5,<0.6
+ordereddict>=1.1,<2.0 ; python_version < '2.7'
diff --git a/setup.py b/setup.py
index 6f0fc17e..446b008e 100644
--- a/setup.py
+++ b/setup.py
@@ -102,28 +102,28 @@ def default_environment():
       maintainer_email='james@hoppipolla.co.uk',
       packages=find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
       install_requires=[
-          'six>=1.9',
-          'webencodings',
+          'six>=1.9,<2.0',
+          'webencodings>=0.5,<0.6',
       ],
       extras_require={
           # A empty extra that only has a conditional marker will be
           # unconditonally installed when the condition matches.
-          ":python_version == '2.6'": ["ordereddict"],
+          ":python_version == '2.6'": ["ordereddict>=1.1,<2.0"],
 
           # A conditional extra will only install these items when the extra is
           # requested and the condition matches.
-          "datrie:platform_python_implementation == 'CPython'": ["datrie"],
-          "lxml:platform_python_implementation == 'CPython'": ["lxml"],
+          "datrie:platform_python_implementation == 'CPython'": ["datrie>=0.5,<2.0"],
+          "lxml:platform_python_implementation == 'CPython'": ["lxml>=2.3,<4.0"],
 
           # Standard extras, will be installed when the extra is requested.
-          "genshi": ["genshi"],
-          "chardet": ["chardet>=2.2"],
+          "genshi": ["genshi>=0.7,<2.0"],
+          "chardet": ["chardet>=2.2.1,<3.0"],
 
           # The all extra combines a standard extra which will be used anytime
           # the all extra is requested, and it extends it with a conditional
           # extra that will be installed whenever the condition matches and the
           # all extra is requested.
-          "all": ["genshi", "chardet>=2.2"],
-          "all:platform_python_implementation == 'CPython'": ["datrie", "lxml"],
+          "all": ["genshi>=0.7,<2.0", "chardet>=2.2,<3.0"],
+          "all:platform_python_implementation == 'CPython'": ["datrie>=0.5,<2.0", "lxml>=2.3,<4.0"],
       },
       )
diff --git a/tox.ini b/tox.ini
index da64de71..06f68a25 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,15 +1,17 @@
 [tox]
-envlist = {py26,py27,py33,py34,py35,pypy}-{base,optional}
+envlist = {py26,py27,py33,py34,py35,py36,pypy}-{base,optional}
 
 [testenv]
 deps =
-  flake8<3.0
-  pytest
+  flake8==2.6.2
+  pycodestyle==2.0.0
+  pyflakes==1.2.3
+  pytest>=2.7.3,<4.0
   pytest-expect>=1.1,<2.0
-  mock
-  base: six
-  base: webencodings
-  py26-base: ordereddict
+  mock>=0.7,<3.0
+  base: six>=1.9,<2.0
+  base: webencodings>=0.5,<0.6
+  py26-base: ordereddict>=1.1,<2.0
   optional: -r{toxinidir}/requirements-optional.txt
 
 commands =