Prolonged sedentary behaviour has established health risks; however many individuals spend long periods of time sitting at work. Qualitative researchers suggest that inconsistent effectiveness of interventions may be linked to an incomplete understanding of the nature of and factors influencing sedentary behaviour in the workplace. Therefore, this study’s aim was to use the COM-B model of behaviour, complimented by the Theoretical Domains Framework, to examine how Capability, Opportunity and Motivation influences sitting behaviour at work in office workers. The study was a qualitative phenomenological analysis in which office workers (n=10) who had predominantly desk-based jobs were interviewed about their sitting behaviour at work. Interview analysis using the Nvivo 10 programme and the Framework Method identified themes and quantified the prevalence of each theme across participants. The analysis incorporated a number of procedures to enhance the trustworthiness including researcher reflexive journal and coding triangulation. Physical capability had minimal influence on behaviour, but psychological capability was influential. Physical and social opportunities were influential in terms of the physical environment, nature of the job, social acceptability and norms related to sitting. Both automatic and reflective motivation emerged as being influential on sitting behaviour. To conclude, the COM-B model, complimented by the Theoretical Domains Framework (TDF), facilitated understanding of factors influencing office workers’ sitting behaviour and highlighted a number of potential areas for future intervention foci.