Voluntary behaviors (i.e., knowledge contribution and word of mouth) are important to the sustainability and growth of online knowledge communities. Although previous studies have identified various factors leading to knowledge contribution and related behaviors, the underlying psychological processes have rarely been examined. In particular, previous studies have not examined how characteristics of online knowledge communities influence voluntary behaviors through support perception. This study aims to fill the gap in the literature by developing and testing a model to explain voluntary behaviors in online knowledge communities. To develop the research model, we drew on theories of justice, organizational support, and citizenship behavior to explain the influence of characteristics of online knowledge communities on individuals' voluntary behaviors through their perceptions of support from the community and the leader. The research model was tested on survey data collected from 214 online knowledge community users. The results largely supported our model. In particular, we found that pro-sharing norm and information need fulfillment affect perceived community support. Perceived recognition from leader and perceived co-presence of leader affect perceived leader support. Additionally, perceived community support was found to be important in shaping knowledge contribution and word of mouth. Perceived leader support was found to influence individuals' knowledge contribution behavior. Theoretical and Practical implications are discussed.