Synchronization has been shown to play an important role in social life through its effects on interactions between people and the quality of these interactions. However, little is known about how observing synchronization affects perceptions of the synchronized individuals. This paper examines how observed synchronization influences perceptions of a neutral person depending on the emotional valence of the faces with which they are synchronized. Two different forms of synchronization were used in these studies: synchronous flashing of faces and faces moving in a common direction. We hypothesized that observed synchronization biases the perception of emotions expressed by a neutral person and an observer’s attitude towards this person. These effects are expected to be congruent with the valence of the synchronizing faces. The results showed a divergent pattern of effects for different forms of synchronization. In Study 1, synchronous flashing biased only the perceived emotions. In Study 2, synchrony of movement affected participants’ attitudes towards the observed person. Our findings suggest that the form of observed synchrony is an important factor in drawing inferences about individuals.