87% of Journals Allow Open Access Archiving
The information comes from SHERPA/RoMEO*, the de facto number one global resource for understanding retained open access sharing rights for journal publications. 87% of journals allow some form of immediate archiving on sites like the Leicester Research Archive (LRA)
There are variations in the version that can be archived. For 60% of cases it is the post-peer review version, generally the author's final submitted version rather than the publisher's PDF. However, while almost 25% of journals allow the publisher PDF to be deposited on the LRA, if you aren't sure what your journal allows you should always deposit your final version in the first instance.
Excitingly the percentage of permitted archiving rises to 94% allowed when embargo and other restrictions are allowed to pass - which means more than ever it is worth depositing articles via IRIS to the LRA.
*The statistics were compiled from a snapshot of the SHERPA/RoMEO Journals database taken on the 15th Nov.2011, when it contained about 19,000 journal titles.