Should participation in employee trainings be mandatory or voluntary? This remains an important matter for organizations aiming at fostering employee development. Previous studies have provided mixed evidence about the merits of mandatory or voluntary training participation for transfer of training. One explanation for this is that these studies did not take differences in training characteristics (i.e., soft- or hard-skills, online or in-person) into account. It also remains unclear how personal and contextual antecedents of transfer of training and transfer motivation differ between mandatory versus voluntary trainings. Departing from the Unified Model of Task-specific Motivation (UMTM), the aim of this study is to investigate the effects of mandatory or voluntary training participation on antecedents of transfer motivation, transfer motivation and transfer of training for different training characteristics. A total of 1122 trainees filled in a questionnaire covering the UMTM components directly after attending a training and 728 trainees indicated transfer of training after 6 weeks. Data were analyzed by means of multi-group structural equation modelling. The outcomes indicate that the effects of mandatory versus voluntary training participation on personal and contextual antecedents and transfer of training depend on whether trainings cover soft- or hard-skills, whereas voluntary training participation is more beneficial for transfer motivation, irrespective of other training characteristics. The findings underline that training content matters for the effects of mandatory and voluntary training participation on transfer of training. This provides guidelines towards trainers and policy makers in which cases mandatory or voluntary training participation is more beneficial.