Background: The aim of this thesis was to provide a systematic examination of the potential of a Verbal Thought Disruption Technique (VTDT) in reducing ruminative and intrusive thoughts (RATs). Thought Suppression (TS) is a commonly used tactic to deal with RATs. In contrast, very little is known about VTDT as a specific cognitive defusion technique in particular, or as a cognitive behavioural technique in general.Methodology: A mixed methods case series design was used for this project. 10 individuals were involved over a 12-week period. An ABACAB design was utilised in which each individual received no intervention (A), a VTDT intervention (B) and a TS intervention (C) over the course of the study. Change was monitored quantitatively utilising the Meta-Cognitive Questionnaire-30 (MCQ-30), Thought Control Questionnaire (TCQ) and the General Health Questionnaire (GHQ). Additionally the measures were also completed at a one-month follow-up alongside the completion of Client Change Interview Protocol. The quantitative data was analysed by calculating the effect sizes of the reported change and the qualitative data analysed by thematic analysis.Results: Use of VTDT appeared to have a positive impact on RATs five MCQ-30 sub-measures (Cognitive Confidence, Need for Control, Cognitive Self-Consciousness, Positive and Negative Beliefs). VTDT use showed a positive impact as measured by the TCQ in four areas Distraction, Punishment Re-appraisal and Worry. The GHQ showed positive effects for VTDT use in the areas of Anxiety and Insomnia, and Social Dysfunction. TS had a negative impact on all measures. VTDT decreased numbers of RATs and severity of Subjective Units of Distress (SUDs) and TS increasing them, indicating a beneficial effect for VTDT and a detrimental effect for TS. The size of this effect was greater on SUDs than on RATs.Qualitative analysis at follow up identified factors that supported the use of VTDT and of the lack of utility of TS protocols to deal with RATs. Moderating factors for the first technique were found in the lack of willingness of some participants to use it in the future. A new working model of RATs was proposed.Conclusions: Conclusions reached are that VTDT may be a useful addition to an already wide range of cognitive defusion techniques already used in Acceptance and Commitment Therapy (ACT), and that it deserves further systematic research. TS, as previously found, appeared not to be a useful technique in dealing with RATs on most of the measures used.