This paper describes an exploratory study of aspects that should be taken into consideration when optimizing tool kits, i.e. cases containing sets of tools that are used by service engineers for corrective maintenance. The study was carried out among service engineers of an Original Equipment Manufacturer (OEM) that produces lithography equipment that is critical in the production process of its customers. First, by means of the Critical Incident Technique, a conceptual model was developed incorporating the aspects that are associated with engineers’ preferences for either separate tools or tool kits in servicing lithography equipment. Next, the actual preferences and most important determinants were established through a survey among engineers worldwide. The results show a preference for tool kits, both in general and for all phases of the maintenance process. Many aspects appear to play a significant role in this preference, such as packing, convenience, characteristics of the repair action, uncertainty, and time. Some of these aspects have been studied in the literature, mainly via quantitative models, while others have not been identified before.