A novel object-oriented methodology for the design of model libraries is exemplified by means of the design of JARA. JARA is a set of libraries of dynamic hybrid models of some fundamental physical-chemical principles. Its main application field is the modeling of physical-chemical processes in the context of automatic control. Only a reduced set of phenomena has been selected for modeling, focusing mainly on illustrating relevant aspects of the proposed design methodology. The five steps of the design methodology are followed and the design rules of JARA are obtained.