Four subjects—ecology, applied mathematics, sociology, and economics—were selected to assess whether there is a citation advantage between journal articles that have an open-access (OA) version on the Internet compared to those articles that are exclusively toll access (TA). Citations were counted using the Web of Science, and the OA status of articles was determined by searching OAIster, OpenDOAR, Google, and Google Scholar. Of a sample of 4,633 articles examined, 2,280 (49%) were OA and had a mean citation count of 9.04 whereas the mean for TA articles was 5.76. There appears to be a clear citation advantage for those articles that are OA as opposed to those that are TA. This advantage, however, varies between disciplines, with sociology having the highest citation advantage, but the lowest number of OA articles, from the sample taken, and ecology having the highest individual citation count for OA articles, but the smallest citation advantage. Tests of correlation or association between OA status and a number of variables were generally found to weak or inconsistent. The cause of this citation advantage has not been determined.