In this paper a knowledge based system (KBS) for reactor selection is discussed. The objective of the KBS is to select the best reactor for a chemical process of known kinetics. The most important step in developing a KBS is to decide which tasks are needed to solve the problem. The main-task reactor selection is decomposed into sub-tasks, according to a task structure. Reactor selection is an integral part of conceptual design of a chemical process which has following consequences for the task structure. A reactor engineer will first use criteria that apply to all selection problems. Then he tries to identify the important concepts of the current selection problem. For other selection problems the set of important concepts may be totally different. It is argued that reactor selection is a creative task since no predetermined set of important concepts exists that applies to each selection problem.