The aim of this study was to examine the degree in which measurements of trait experiential avoidance (EA) are affected by current emotional disorder and whether EA is a causal factor in the course of emotional disorders (anxiety and depressive disorders) and the development of comorbidity among emotional disorders. In a sample of 2,316 adults aged 18 to 65, consisting of healthy controls, persons with a prior history of emotional disorders, and persons with a current emotional disorder, DSM-IV-based emotional disorders (CIDI: Composite Interview Diagnostic Instrument) were assessed at T2 and 2 (T4) and 4. years later (T6) and experiential avoidance (AAQ: Acceptance and Action Questionnaire) at T2 and T4. Results showed that EA scores were stable over a 2-year period notwithstanding state fluctuations because of current emotional disorder. Moreover, EA scores at T2 predicted changes in distress (major depressive disorder, dysthymia, generalized anxiety disorder) and in fear disorders (social anxiety disorder, panic disorder with or without agoraphobia, agoraphobia without panic) at T4. Finally, EA at T4 mediated the longitudinal association of fear disorders at T2 with distress disorders at T6 as well as of distress disorders at T2 with fear disorders at T6. These findings suggest that EA scores are more than epiphenomena of emotional disorders and that EA may be conceptualized as a relevant transdiagnostic factor affecting the course and development of comorbidity of emotional disorders.