Background: Comorbidity between social anxiety disorder (SAD) and substance use disorder (SUD) is elevated but little is known about the role of avoidance with regard to the aetiology and maintenance of these disorders. Aims: The objective of the study was to compare patients with SAD and comorbid SUD with patients suffering from SAD only with regard to fear and avoidance symptoms. Methods: Thirty patients presenting SAD and comorbid SUD were recruited by the local Substance Abuse Unit, and 26 patients with SAD only were identified within the general population by the research team. All patients were asked to answer the Liebowitz Social Anxiety Scale. Results: Patients suffering from SAD and SUD showed a higher avoidance/total score ratio. Avoidance contributed at mean to 49.7% (±4.9%) of the total score in patients with SAD and SUD, whereas its mean contribution was 46.5% (±3.1%) in patients with SAD only (effect size 0.77). Conclusions: Avoidance tendencies appear to be more prominent in patients suffering from SAD and SUD. Focusing on avoidance may be of interest for clinical practice and for research when investigating the cognitive and behavioural bottom structure of patients suffering from SAD and comorbid SUD.