Organizational performance improves through several channels, including changes in efficiency, innovation and technological change. Most of the extant research has focused on overall performance, often measured by partial measures of productivity, with little attention given to the components of performance. The aim of this paper is to analyze the impact of HR practices and unionization on one important channel – organization efficiency - as measured by technical and scale efficiency. Using French industry survey data, the paper shows that HR practices do influence efficiency, but this is moderated by the existence of unions. The results show a rather complex set of associations. We find robust results that show that in France, HR practices have a positive effect on scale efficiency but this effect is dampened in the presence of unions. On their own, HR practices have no effect on technical efficiency. However, some of the results suggest that HR practices can exert a positive influence when combined with unions.