This paper studies how employees anticipate change in social relations after a strike in their organisation. We hypothesise that two group norms affect employees' fear that a strike deteriorates the social relations with their colleagues. We distinguish between two different norms associated with a collective action problem such as a strike. A ‘solidarity norm’ prescribes that workers should participate in collective protest when called for. A ‘free rider punishment norm’ describes whether and how violators of the solidarity norm should be treated. While the former itself theoretically does not impact intra-group social relations, the latter one does. We test our hypotheses on a data set of 468 Dutch union members. Adherence to the free rider punishment norm significantly increases a fear for deterioration of social relations with colleagues after a strike, while adherence to the solidarity norm does not. We also find a moderating effect of expected proportion of strikers. The results are discussed with reference to industrial relations theory and mobilisation theory, further probing into mechanisms that relate social norms to the (anticipated) costs of participation and the quality of social relations after a strike.