Abstract The triad is one of the most basic human groups in social networks. Understanding factors affecting the formation of triads will help reveal the underlying mechanisms that govern the emergence and evolution of complex social networks. In this paper, we study an interesting problem of decoding triadic closure in social networks. Specifically, for a given closed triad (a group of three people who are friends with each other), which link was created first, which followed, and which link closed. The problem is challenging, as we may not have any dynamic information. Moreover, the closure processes of different triads are correlated with each other. Our technical contribution lies in the proposal of a probabilistic factor graph model (DeTriad). The model is able to recover the dynamic information in the triadic closure process. It also naturally models the correlations among closed triads. We evaluate the proposed model on a large collaboration network, and the experimental results show that our method improves the accuracy of decoding triadic closure by up to 20% over that of several alternative methods.