Gender equality at work has become in recent years a priority for governments. In France, collective bargaining is a main lever to achieve progress on gender equality issues. In a two-tier bargaining framework, industries and firms are required by law to negotiate on the reduction of gender inequalities. Using firm-level survey data on labor relations issues combined with administrative data, this paper seeks to better understand the dynamics of collective bargaining on gender equality at the firm level by questioning the role played by the gender mix. We find that gender diversity favors gender equality bargaining at the firm level. Under-representation and over-representation of women reduce the probability of firms negotiating an agreement on gender equality. The introduction of sanctions in the recent period has prompted low-feminized firms to negotiate more on gender equality but had little impact on highly feminized firms.