Aim: This thesis describes the development and initial validation of a questionnaire to measure Hayes, Strosahl and Wilson’s (1999) constructs of cognitive fusion and cognitive defusion. Within the literature there is currently no specific measure of these constructs. Design and Method: Principal Component Analysis was conducted on two independent samples (Study One n = 425 and Study Two n = 167). Reliability analyses were conducted for both Study One and Study Two and validation analyses were conducted in Study Two. All participants in both studies completed the Cognitive Fusion Questionnaire (CFQ). Participants in Study Two completed additional measures related to their satisfaction with life, their beliefs about worry, mindful responding to unpleasant thoughts and images and levels of experiential avoidance. Results: The final solution revealed a two component fifteen item questionnaire accounting for 54% of the variance. Based on item content, the components were labelled fusion and defusion. The items within the questionnaire reflected Hayes et al. (1999) constructs of cognitive fusion and cognitive defusion. Internal consistencies as measured by Cronbach’s alpha were .91 (fusion), .71 (defusion) and .88 (total scale). The measure correlated moderately to highly and in the expected directions with questionnaires measuring individual beliefs about worry, mindful responding to unpleasant thoughts and images and levels of experiential avoidance. Similarly, there was a significant negative correlation between the current questionnaire and a measure related to satisfaction with life. Conclusions: The findings of the above research provide initial support for the CFQ. The results show support for the validity of the scale including content and convergent validity of the CFQ.