This article describes the process evaluation of a stress management program called 'Learn Young, Learn Fair' for 5th and 6th graders. Studies, reviews and meta-analyses of prevention programs report that a common limitation in studies is the restricted documentation of process factors that contribute to the success of interventions. Program implementation is such a factor and findings indicate that the quality of implementation significantly affects outcomes. It is associated with Type III error: no effect is established because the program itself is inadequately designed or delivered. The potential role for school psychologists in process evaluation is discussed, and the great deal Intervention Mapping has to offer in this respect. A questionnaire for both teachers and pupils was developed, with items formulated to assess implementation and to investigate whether teachers and pupils valued the program. The latter would give an indication of future adoption and implementation in the field. Implementation was divided into completeness and fidelity. Completeness for the main parts of the program, the lessons and homework assignments, was good (98.5 and 95.8 percent respectively). Completeness for the auxiliary components, the booster sessions and the daily practice exercises, was moderate (62.2 and 69.3 percent respectively). Fidelity was good; mostly the teacher implemented the lessons, almost all items were discussed, and the manual was quite strictly followed (94, 98 and 95 percent respectively). Implementation of the program was of good quality. The program was delivered as intended. Effect evaluation will not be threatened by a Type III error. The results of this study favoured the feasibility of program implementation. It is stated that school psychologists can use the findings as a criterion for program selection. In that case it will be an example of the process of enlarging the potential role of school psychology in process evaluation of mental health interventions for schools.