For analysing field data as well as for modelling purposes it is useful to classify tree species into a few functional types. Inthis paper a new aggregation of tree species of the dipterocarp rain forests in Sabah (Borneo), Malaysia, is developed. Theaggregation is based on the two criteria successional status and potential maximum height. Three classes of successionalstatus (early, mid and late successional species) and five classes of potential maximum heights (<=5 m, 5-15m, 15-25m,25-36m, >36 m) lead to a combination of 15 functional types. The criteria of the developed classication are chosen to suit forapplications with process-based models, such as FORMIX3 and FORMIND, which are based on photosynthesis productionas the main process determining tree growth. The concept is universal and can easily be applied to other areas. With thisnew method of grouping a more realistic parametrization of process-based rain forest growth models seems to be possible.