Social anxiety and depression are highly comorbid conditions. Although this has led researchers to suggest that social anxiety may convey risk for depression, few studies have examined mechanisms of comorbidity between these two conditions. The current study tested two mechanisms of comorbidity between symptoms of these disorders: brooding and excessive reassurance seeking. The data were evaluated in an undergraduate sample assessed three times over a 2 month period. Results suggested that brooding mediated the relationship between social anxiety at Time 1 and depression at Time 3, although no evidence was found for excessive reassurance-seeking as a mediator of these two variables. These findings support recent research suggesting that brooding may be a transdiagnostic process. Theoretical and clinical implications are discussed.