Objectives: Infidelity, actual or suspected, can trigger strong emotional reactions, such as jealousy, which could lead to the dissolution of the relationship. These reactions were predicted to vary with the severity of the infidelity, with the sex of the participant, with previous experience with unfaithful partners, and with the length of the relationship. Method: We employed a sample of 447 Greek-speaking participants who were asked to indicate their reactions in different scenarios of infidelity. Results: We found that more severe acts of infidelity were associated with higher emotional upset and jealousy and lower probability of forgiveness. Moreover, women indicated stronger emotional upset and jealousy than men, but they were more likely to forgive their partners. Furthermore, participants indicated more emotional upset and jealousy if they were in a long-term than in an early-stage relationship. Finally, participants who were older and who had experienced infidelity from their previous partners were more likely to forgive their partners’ infidelity than participants who were younger and who did not have such experience. Conclusions: Our results indicated that several factors determined the severityof the reactions to infidelity.