Companies using social media as communication channels are susceptible to negative word-of-mouth (WOM), which can take extreme forms (also referred to as ‘firestorms’) and may have important consequences. In this article, we aim at modelling the spread of negative WOM in online social networks and investigating the conditions that cause firestorms to emerge. We identify the determinant factors that are related to the spread of negative WOM and examine their impact by proposing an innovative analytical model that offers quantitative insights. In addition, social media also provide companies the possibility to outweigh negative WOM by responding with positive WOM. We present a thorough empirical evaluation of our model, whose findings support decision-making about assessing the risk and the necessity of a response, the required time for it and the effectiveness of procedures for selecting social network members who can spread positive WOM.