In order to improve our understanding of the relationships between high-performance work systems and firm performance, several studies have analysed the mediating effects of motivation-related or human capital-related variables. However, most of these have concentrated on single aspects and are US-focused. We extend previous human resource management research by simultaneously analysing the relevance of four general mediating mechanisms: human capital, employee attitudes, employee performance and operational performance. We apply structural equation modelling with formative constructs to data of 1099 German firms. While our findings support the assumption of positive relationships between high-performance work practices, the four mediating mechanisms and firm performance, they also reveal some peculiarities attributable to the German context. Using formative constructs, we were also able to show that single high-performance work practices have different effects on firm performance.