Adopting the framework of social exchange in researching supportive communication, this work proposes the concept of support exchange. This dissertation defines support exchange and discusses how support exchange is similar to and different from instrumental exchange, which has been the main topic the previous social exchange research. Based on the discussion, the propositions on the relationship between exchange structure and social solidarity in support exchange are established. The propositions were tested through a controlled small-group lab experiment with a 2 (exchange structure: direct reciprocal exchange vs. generalized exchange) x 2 (support type: emotional vs. informational support) design. 285 participants embedded in 95 triads interacted in an online support group following the instructions. Findings from the experiment and the linguistic analysis of the interaction data show that direct reciprocal support exchange generates stronger social solidarity, perceived supportiveness, and commitment to the online group than generalized support exchange does. Limitations and implications of the findings are discussed.