Research indicates an effect of nonverbal synchrony on the therapeutic relationship and patients' symptom severity within psychotherapy. However, vocal synchrony research is still rare and inconsistent. This study investigates the relationship between vocal synchrony and outcome/attachment dimensions, controlling for therapeutic alliance and movement synchrony. Our sample consisted of 64 patients with social anxiety disorder. Symptom severity was assessed with the Liebowitz Social Anxiety Scale and the Inventory of Interpersonal Problems, whereas attachment was assessed with the Experiences in Close Relationships Questionnaire at the beginning and end of therapy. Therapeutic alliance was measured with the Helping Alliance Questionnaire II. We determined vocal synchrony of the median and range of the fundamental frequency (f 0) by correlating f 0 values of manually segmented speaker turns. Movement synchrony was assessed via motion energy and time-series analyses. Patient- and therapist-led synchrony was differentiated. Statistical analyses were performed using mixed effects linear models. Vocal synchrony had a negative impact on outcome. Higher vocal synchrony led to higher symptom severity (if the patient led synchrony, at the end of therapy) as well as attachment anxiety, avoidance, and interpersonal problems at the end of therapy. Predicting attachment anxiety, the effect of therapist-led vocal synchrony went beyond the effect of therapeutic alliance and movement synchrony. High vocal synchrony may arise due to a lack of autonomy in social anxiety disorder patients or might reflect attempts to repair alliance ruptures. The results indicate that vocal synchrony and movement synchrony have different effects on treatment outcome. (PsycInfo Database Record (c) 2021 APA, all rights reserved).