A computer aided modeling system, ModDev, is presented. ModDev is a process-modeling tool that assists in the derivation of model equations for a process module (unit operation), in the analysis and mathematical manipulation of the derived model equations, and in translation of the model equations with a process simulation system such as ICAS, or gPROMS. This paper describes the theoretical basis for the modeling system, the main features of the computer aided modeling system, and highlights the derivation and solution of model equations through two examples involving a reactor and a distillation column.