Citizen science projects provide non‐scientists with opportunities to take part in scientific research. While their contribution to scientific data collection has been well documented, there is limited research on how participation in citizen science projects may affect their scientific literacy. In this study, we investigated (1) how volunteers' attitudes towards science and epistemological beliefs about the nature of science changed after six months of participation in an astronomy‐themed citizen science project and (2) how the level of project participation related to these changes. Two main instruments were used to measure participants' scientific attitude and epistemological beliefs and were administered before they registered for the program and six months after their registration. For analysis, we used pre‐ and post‐test data collected from 333 participants who responded to both tests. Among them, nine participants were randomly chosen for interviews. Participants' responses were analyzed using the Rasch Rating Scale Model. Results show that overall scientific attitudes changed positively, p < 0.01. The change was strongest in attitudes towards science news and citizen science projects. The scientific attitudinal change was related to participant social activity in the project. There was a negative change in their evaluation of their knowledge. The interviews suggest that this is due to a greater appreciation for what they have yet to learn. Epistemological beliefs about the nature of science significantly improved from the pre‐ to the post‐tests, p < 0.05. Overall, we found volunteers' participation in social components of the program was significantly related to their improvement in scientific literacy while other project participation variables (such as amount of data contributed to the project) was not. © 2013 Wiley Periodicals, Inc. J Res Sci Teach 50: 773–801, 2013