In order to improve our understanding of mediating variables inside the ‘black box’ of the firm's labour management, this paper examines the relationship between high-performance work system (HPWS) practices and employee attitudes. Using a randomly selected, national population sample, clear evidence was found for a positive relationship between HPWS practices and the attitudinal variables of job satisfaction, trust in management, and organizational commitment, implying that HPWS can provide win-win outcomes for employees and employers. However, the study also tests – from an employee perspective – the ‘complementarities thesis’ and finds negative interaction effects among HPWS practices. This strengthens the argument that there are likely to be limits to the positive outcomes of HPWSs for employees. Evidence of sequencing in the employee attitudinal responses to HPWSs was also found, with job satisfaction as the key mediating variable.