Sedentary behaviour (SB) is highly prevalent within desk-based employees (Clemes, O'Connell, & Edwardson, 2014) and is associated with a number of negative health consequences, including cardiovascular disease and diabetes (Wilmot et al, 2012). Interventions to reduce SB in the workplace, including active workstations (AWSs), can reduce SB (Neuhaus et al, 2014a), yet initial reductions are not to be sustained beyond 12-months (Koepp et al, 2013). Moreover, equipment-based interventions (e.g. AWS's) are perhaps unaffordable for many areas of commerce and industry. Understanding the determinants of SB in the workplace could help in the design of pragmatic, scalable interventions to maintain reductions in SB. A thorough exploration of the determinants of SB in desk-based workers was undertaken using an online questionnaire (Study 1, n=1,101), and semistructured interviews (Study 2, n=14). Organisational norms, control to sit, intentions to sit, social influences, and awareness of SB were identified as determinants of SB, underlining the complexity of SB in the workplace. Sitting was also reported to be a habitual behaviour and interestingly participants with AWSs only sat for 36 minutes less/working day than those with fixed sittingheight desks. This observation is low compared to data from previous research (78 minutes; Neuhaus et al, 2014a).