Online networks can be construed as social networks in which people engage in interactions, build rela- tionships, share information, and request and extend assistance to each other using electronic communication technologies. Is social capital embedded in online networks? How is such social capital mobilized (i.e., shared)? What structural properties of cyber networks are associated with mobilization of social capital? These questions have drawn the attention of researchers in the areas of social networks, social capital, and online communication. Our research is an initial effort to touch upon these three questions. Whereas most previous research on both conventional and online social networks has favored analysis of either egocentric (i.e., individuals) or bounded (i.e., groups or organizations) networks as the primary unit of analysis, this study investigates online discussion forums that span formal boundaries of organizations, examining their structural properties and patterns of information exchange. We employ a network-based approach to the study of social capital, postulating that mobilization of social capital is contingent on social network properties. Using data from professional online forums devoted to knowledge management, we find that two network properties, core–periphery structure and centralization, are related to the mobilization of informational social capital in online networks. Implications and limitations of this study are discussed and suggestions for future research are provided.