The paper examines if workplace gender diversity offers some explanation for the decline of unions in Britain. Using the WERS2004 linked employer-employee data and alternative econometric estimators it reports an inverse relationship between workplace union density and gender diversity. Gender and ownership status based sub-group analyses suggest the inverse relationship to be stronger for male union members and those in the private sector. Gender group size based analysis reveals a positive link between workplace union density and gender diversity in workplaces with a female majority. The findings in this paper may mean that unions (and their main constituents, men) may need to embrace the changing workplace demography genuinely to improve their fate.