While many studies of interventions have focused on their content and immediate effects, less research has focused on the processes that may explain these effects. The purpose of this study was to show how process evaluation can be used to interpret the results of an intervention study in four industrial canteens in Denmark. Two canteens acted as intervention groups and two as comparison groups. Effects were measured by surveys before and after interventions, and observations and interviews were conducted to provide an in-depth understanding of processes. Analyses were conducted based on the responses from 118 employees. Results showed, contrary to expectations, improvements in working conditions and well-being in one intervention group and in one comparison group, whereas no improvements were found in the two remaining groups. Data from the process evaluation enabled a meaningful interpretation of these results, raising the possibility programme failure rather than theory failure, and thereby constituting an example of how process evaluation can shed light over the factors that may influence outcomes in controlled intervention studies.