1. Impact of the Toll-access vs. Open-access Resources

2. ISI Impact Study on Open Access Journals "Of the 8,700 selected journals currently covered in Web of Science, 191 are OA journals... [A study on] whether OA journals perform differently from other journals in their respective fields [found] no discernible difference in terms of citation impact or frequency with which the journal is cited".http://www.isinet.com/oaj

3. ISI OA Impact Study Impact Study of OA in Physics

4. Impact Study of OA in Computer Science Lawrence found that in computer science citations were three times higher for Open Access articles than for papers only available for payment in print or online.

5. Impact Study of OA in Mathematics Odlyzko reported similar estimates in mathematics