The Adler Planetarium, Johns Hopkins University, and Southern Illinois University-Edwardsville are investigating the potential of online citizen science projects to broaden the pool of volunteers who participate in analysis and investigation of digital data and to deepen volunteers' engagement in scientific inquiry. The Investigating Audience Engagement with Citizen Science project is administering surveys and conducting case studies to identify factors that lead volunteers to engage in the astronomy-focused Galaxy Zoo project and its Zooniverse extensions. The project is (1) identifying volunteers' motivations for joining and staying involved, (2) determining factors that influence volunteers' movement from lower to higher levels of involvement, and (3) designing features that influence volunteer involvement. The project's research findings will help informal science educators and scientists refine existing citizen science programs and develop new ones that maximize volunteer engagement, improve the user experience, and build a more scientifically literate public.