PurposeOstracism is a common challenge in the workplace, but little is known about the behaviours of those who trigger it. The authors examined how leader favouritism can drive coworkers to ostracise one another, given that leadership is a key factor in shaping employee attitudes and behaviour. Invoking social comparison theory, the authors assessed a model of how perceived favouritism affects ostracism through jealousy, moderated by organisation-based self-esteem (OBSE).Design/methodology/approachThe authors surveyed 294 non-managerial employees from several service organisations in the United Arab Emirates (UAE) and analysed the data with partial least squares structural equation modelling (PLS-SEM) using SmartPLS 3 software.FindingsUnexpectedly, leader favouritism did not directly affect ostracism but only indirectly through the mediation of jealousy. OBSE was found to moderate this relationship, suggesting that higher levels of OBSE can weaken the impact of leader favouritism on employee jealousy.Originality/valueThese findings explain the intricate dynamics and underlying reasons as to how leader favouritism can instigate employee-to-employee ostracism.