The aim of this paper is to examine the mediating role played by five key job characteristics in the relationship between employee participation in a job redesign intervention and employee well-being. In studies of job redesign interventions, it has been assumed that any effects of employee participation in job redesign on well-being are a result of changes in job characteristics rather than participation in change per se. It is therefore important to statistically test for mediation in job redesign intervention studies to help establish that the change in job characteristics is the mechanism through which job redesign interventions work. However, this has rarely been tested directly, either because data to allow tests of mediation have not been collected (e.g. assessments of job characteristics) or because data have been collected but mediation has not been tested using accepted procedures. This makes it unclear whether changes in job characteristics explain the effects. Results from multilevel analyses of a longitudinal 9-month long serendipitous quasi-experimental participative job redesign intervention showed that changes in job control, participation, skill utilization and feedback, but not task obstacles, were sufficient to account for the relationship between the intervention and employee well-being. Copyright © 2009 John Wiley & Sons, Ltd.