Objective: The aim of the current study was to compare two clinical groups on attachment, alexithymia and the therapeutic alliance, as well as investigate the potential mediating influence of alexithymia on the relationship between attachment and the therapeutic alliance. Method: Data was analysed for 203 individuals; 44 attending therapy for an addiction related problem, 59 attending therapy for complex trauma and 100 healthy controls. All participants completed self-report measures on Attachment style (ASQ) and Alexithymia (TAS-20). The addictions and trauma groups also completed a measure of Therapeutic Alliance (W AI). Results: No significant differences were found between the addictions group and the trauma group with respect to attachment and the therapeutic alliance. Both the addictions group and the trauma group reported significantly greater insecure attachments compared to healthy controls. For the addictions group, preoccupied attachment emerged as the significant attachment variable related to the therapeutic alliance. This relationship was partially mediated by alexithymia. Conclusions: The findings have implications for understanding the role of attachment and alexithymia in influencing the development of a positive alliance in individuals across the addiction and complex trauma clinical populations.