While the study of e-participation has gained increasing attention within political science, our understanding of its underlying structure and relationship to offline participation is limited. This article addresses these gaps by focusing on three interrelated questions: (1) Is e-participation a multidimensional phenomenon (differentiation hypothesis)? (2) If submodes exist, do they mirror existing modes of participation (replication hypothesis)? (3) If offline forms are replicated online, do they mix together (integration hypothesis) or operate in separate spheres (independence hypothesis)? We test our hypotheses through confirmatory factor analysis of original survey data from the U.K. General Election of 2010. The results show that distinct submodes of e-participation, comparable to those occurring offline, can be identified. Support for integration and independence varies according to the type of participation undertaken. Finally our results suggest that the online environment may be fostering a new social-media-based type of expressive political behavior. © 2013 Southern Political Science Association.