The mixed empirical findings to date have indicated that some, but not all, unions in industrialized countries are actively involved in campaigning and bargaining around work-life balance (WLB) issues, as part of a modernization agenda linked to feminization and to 'positive flexibility'. This article seeks to identify factors that might encourage or inhibit trade unions from involvement in WLB issues, within a cross-national comparative perspective focusing on two countries (France and the UK) that have contrasting working time regimes and approaches to WLB. It draws on original research carried out in two sectors - insurance and social work - in these two countries. The article links the emergence of union WLB programmes and bargaining agendas to gender-equality concerns within the union and to the gender composition of the sector, as well as to the working time regime, including the mode of action, partnership being a significant corollary of WLB campaigning in the UK. We find support for the modernization thesis in the UK, particularly in the public sector, but within severe constraints defined by employer initiative.