A huge increase in usage of online communities is seen over the last decade. Many online communities face a sustainability problem due to a low level of knowledge contribution. Zhang et al. (2012) proposes that every community is dependent on a relatively small number of people who are a “critical mass” supplying knowledge whereas the rest are free-riding on others’ effort. The factors which influence knowledge supply are a main focus of master thesis. Perceived informational value, commitment to community, reputation and shared language were examined as a direct effect on knowledge contribution in main message and comments separately. Additionally, a moderating effect of shared language was tested. The research was conducted in four different Philips online communities based on different used shared language. It was found that all variables were significantly important for both posting and commenting.However, several interaction effects were insignificant or the influence was contrary to expected. The length of membership which was tested as a control variable was found to be insignificant whereas almost no differences among communities were foundexcluding one exception. Members of Digital Junkies group where the content is mostly IT related contribute less messages than Accelerate! but do not differ for commenting activities.