Thailand, one of the fast growing countries in Asia, has initiated and implemented a series of national plans and activities to promote e-commerce adoption in both public and private sectors. Despite such efforts, e-commerce adoption rate is still slow. It is therefore important to understand factors affecting a firm's decision on e-commerce adoption in Thailand. This study examines the factors influencing e-commerce adoption decisions in small and medium enterprises (SMEs) in Thailand. We classify firms into three main groups based on the earliness of e-commerce adoption, namely adopters, prospectors, and laggards. Three groups of factors influencing adoption decisions are identified, including organizational, technology, and environmental factors. Data was collected through a national survey in Thailand. The statistical analysis results strongly support the hypotheses. The results are interpreted and the implications of this study are subsequently discussed. [PUBLICATION ABSTRACT]