The current thesis delineates a programme of research that utilised the Implicit Relational Assessment Procedure (IRAP) as a means of measuring relational responding that is relevant to obsessive-compulsive disorder (OCD). A total of ten studies were conducted with both clinical and non-clinical adult participants (N = 344). Specifically, the research began with the assessment of relating behaviour pertaining to disgust propensity (initial feeling of disgust) and disgust sensitivity (appraisal of the initial feeling) and sought to determine whether this behaviour was relevant to obsessive-compulsive tendencies and overt avoidance behaviour (n=33). Disgust sensitivity was found to predict avoidance behaviour while disgust propensity was not, whereas both propensity and sensitivity were related to obsessive-compulsive tendencies (obsessing and washing concerns respectively). In order to further develop and refine the IRAP as a measure of OCD, Chapter 3 outlined two studies that aimed to assess whether small changes to the IRAP procedure impacted upon the D-IRAP scores (n=66). Furthermore, Chapter 4 sought to determine whether an inability to disengage from fearful or anxiety-inducing stimuli affected accuracy and response latency on the IRAP (n = 32). Minor changes to the IRAP procedure were not found to affect the D-IRAP score, whereas the extent to which an individual can disengage from anxiety-inducing stimuli was predictive of accuracy on the IRAP but did not affect response latency. With these methodological issues addressed, a series of studies (n=117) that focused on exploring the six obsessive belief domains of OCD, which were first conceptualised from the cognitive-behavioural literature, from a functional perspective using the IRAP were also outlined. The penultimate experimental chapter focused on a behaviour, which was termed Intolerance for Causing Mess (ICM) by placing emphasis on responding to the non-disgusting or pleasant stimuli, and how this may be related to unwillingness to cause mess (n = 36). The research culminated with a study comparing responding on the IRAP between clinical versus control participants (n = 34) and also a study that explored the predictive validity of the IRAP for treatment outcome (n=26). Broadly speaking, one of the most important findings from the current thesis was that the appraisal of the initial feeling of disgust appeared to be most critical to the aetiology of OCD as it predicted overt avoidance behaviour as well as distinguishing between clinical versus control groups. Critically, the appraisal of both negative and positive stimuli was deemed to be significant in the aetiology of OCD. Furthermore, the results of the final study suggested that the IRAP may have predictive validity for short-term treatment outcome that is greater than that of the most widely used self-report measures such as the Padua Inventory-Revised and Yale-Brown Obsessive Compulsive Scale. The present results support the use of the IRAP as a measure of OCD and specifically support the supposition that the appraisal is key in the aetiology and maintenance of OCD. Finally, the results from the current thesis have scope to add to the literature pertaining to contextual behavioural science as well as to the cognitive-behavioural literature.