It has been the matter of much debate whether perceivers are able to distinguish spontaneous vocal expressions of emotion from posed vocal expressions (e.g., emotion portrayals). In this experiment, we show that such discrimination can be manifested in the autonomic arousal of listeners during implicit processing of vocal emotions. Participants (N = 21, age: 20-55 years) listened to two consecutive blocks of brief voice clips and judged the gender of the speaker in each clip, while we recorded three measures of sympathetic arousal of the autonomic nervous system (skin conductance level, mean arterial blood pressure, pulse rate). Unbeknownst to the listeners, the blocks consisted of two types of emotional speech: spontaneous and posed clips. As predicted, spontaneous clips yielded higher arousal levels than posed clips, suggesting that listeners implicitly distinguished between the two kinds of expression, even in the absence of any requirement to retrieve emotional information from the voice. We discuss the results with regard to theories of emotional contagion and the use of posed stimuli in studies of emotions.