This research is one of the first few to investigate the acceptance of open source software in the context of developing countries, in this case Thailand. It also proposes a comprehensive model that integrates individual, social, and software characteristics to measure the acceptance of open source software. Based on the three-pronged approach, the study puts forward an integrated model consisting of the extended technology model, the innovation diffusion theory and software characteristics. A full-scale field survey of 1,005 open source software users was undertaken and both convergent and discriminant validities were conducted. The results of the structural equation modeling illustrate that perceived ease of use, perceived usefulness, intention to use, output quality, compatibility, software functionality, and software maintainability have a statistically positive relationship with open source software acceptance. Contrary to most studies, which did not find a positive relationship between visibility and trialability towards the acceptance of open source software, it is interesting to observe that this study did find a positive relationship. Conclusions and discussions of the findings, and their academic and practical implications are also presented.