Abstract: The growing popularity of Open Source Software (OSS) as an organisational Information Technology (IT) infrastructure has made it important to explore and understand the key determinants of its adoption by Small Businesses (SMBs). Existing research of OSS adoption have ignored the relevance of applying widely accepted theories in the field of Information Systems (IS). This study presents an extended version of the Decomposed Theory of Planned Behaviour (DTPB) for modelling the adoption of OSS by SMBs. That framework is based on an augmentation and critical analysis of the literature of OSS adoption and the adoption of Information and Communication Technology (ICT) in small businesses and an appraisal of relevant theories for organisational adoption for of ICT. We show that our proposed framework provides useful concepts for identifying factors and advanced valid explanations of their influences on the adoption of OSS. We discuss the useful implications of the framework from the perspectives of research and practice, within the contexts of direct utilisability, foundation for justification and general frame of reference. The dearth of research in the area of OSS adoption has suggested that this study is among the first to develop a theory-grounded framework for the evaluation of OSS adoption in small businesses.