Research on group identification has shown it to be a surprisingly weak predictor of intentions to take large-scale social action. The weak links may exist because researchers have not always examined identification with the type of group that is most relevant for predicting action. Our focus in two studies (one in Romania and one in Australia, both Ns = 101) was on opinion-based groups (i.e. groups formed around shared opinions). We found that social identification with opinion-based groups was an excellent predictor of political behavioural intentions, particularly when items measuring identity certainty were included. The results provide clear evidence of the role of social identity constructs for predicting commitment to social action and complement analyses of politicised collective identity and crowd behaviour. Copyright © 2006 John Wiley & Sons, Ltd.