Purpose: The purpose of the study was to analyse the role that high-performance human resource management practices (training and development opportunities, development-oriented performance appraisal, information sharing, participation opportunities and reward practices with performance-based incentives) play as predictors of organisational affective commitment. It aims also to analyse the mediating role played by organisational affective commitment in the relationship between these practices and spontaneous work behaviour favourable to organisations; more specifically, making suggestions and disseminating a positive image of the organisation, helping co-workers and showing a concern for quality of work and making a personal investment to their professional development. Methodology: The study was based on a questionnaire answered by employees (n=514) at six Portuguese banks. The predictive variables (high-performance human resource management practices), affective commitment and the criterion variables (spontaneous work behaviours favourable to the organisations) were measured using items based on research literature. We conducted a factorial exploratory analysis and we used multivariate statistical procedures. Findings: According to the results training and development practices, development-oriented performance appraisal, information sharing and rewards practices with performance-based incentives are predictors of affective commitment. The results revealed also that affective commitment contributes to spontaneous work behaviour favourable to organisation (making suggestions and disseminating a positive image of the organisation, helping co-workers and showing a concern for quality of work). They also suggest a virtuous relationship between affective commitment and training, as they show that training and development practices contribute positively to affective commitment and that affective commitment contributes to personal investment in professional development. Relevance: The paper contributes to explain the relationship between high performance human resource management practices (HP HRMP) and organisational results; we suggest indicators to build a scale of HP HRMP and we stress the importance of considering a multidisciplinary framework in order to explain the relationship between HP HRMP and organisational results.