Self-harm and externally-directed impulsive behaviours are relatively common in psychiatric populations. Different explanations have been advanced for the presence of these behaviours, including the suggestion that they are driven by unconditional beliefs (schema-level cognitions) and that they are related to dissociation (a reduction in processing of intolerable cognitions and affect). This brief study begins to test a model based on the hypothesis that unconditional core beliefs are associated with the use of impulsive behaviours, and that dissociation is a key factor that mediates the relationship. An unselected group of 50 psychiatric inpatients completed standardized self-report measures of core beliefs, dissociation and impulsive behaviours. For female patients only, the results of mediational multiple analysis were compatible with a model where the relationship between unconditional abandonment beliefs and self-harming behaviours is perfectly mediated by dissociation. The strengths, limitations and directions for further research are discussed.