Many theoretical models have been proposed to explain the relationship between severe mental illness (SMI) and substance use. Because many of these are contradictory quantitative American studies, a qualitative, exploratory study of a Scandinavian sample may offer a new perspective. The aim of the study is to explore reasons for substance use through analysis of the participants’ experiences. A qualitative study with semistructured interviews was used. Purposeful sampling (N=11) of patients with substance use disorder (SUD) and SMI, who were included in assertive community treatment teams, was completed. Inclusion criteria are increased quality of life or increased general functioning, and decreased substance use, after a minimum of 12 months in treatment. Reasons given for using substances were categorized as (a) controlling the symptoms of mental illness, (b) counteracting medication side effects, or (c) balancing the ambiguity. The conclusion is that the study findings mainly support secondary substance use models in explaining the comorbidity of SMI and substance use. However, there is some support for the traditional self-medication hypothesis (SMH), iatrogenic vulnerability, and the supersensitivity model. This may be because the majority of the study participants reported having a mental illness with subsequent substance use. The expressed ambivalence to substance use also lends some support to bidirectional models.