This article provides empirical evidence to support recent assertions that the substantive representation of women depends not only on the numbers of women elected representatives in national legislatures, but also who they are. In this case study of one of the UK's devolved legislatures, analysis was undertaken of the transcripts of 327 plenary debates held during the first term of the National Assembly for Wales, where women constituted 42 percent of elected members (1999–2003). The gender dynamics of political debate around key equality topics reveal that the link between descriptive and substantive representation of women is complex. When a ‘critical mass’ of women is achieved the substantive representation of women is affirmed as ‘probabilistic’ rather than ‘deterministic’ for it is shaped by the institutional context, the gender dynamics of debate and, importantly, the actions of individual ‘equality champions’. While women representatives exhibited a greater propensity to advance gender equality in debate than their male colleagues, the present findings also show the disproportionate influence of ‘equality champions’: women who are able to draw upon earlier feminist activism and act as ‘strategic insiders’ who make a difference to women's issues in a parliamentary context.