Reaction and separation in situ brings synergic effect of various phenomena (kinetic, heat and mass transfer) and phase equilibria. When modelling such a process, various types of models can be used. The goal of this paper was to create a non-equilibrium mathematical model of a reactor, in which reactive distillation or absorption with fast chemical reaction takes place. To characterize the vapour-liquid contact, film layer theory was used. In the presented paper, Fick's equation is used to describe mass transfer with reaction in the liquid film. It is suitable for systems where a fast reaction occurs in the liquid film. Simulation of a steady-state reactive distillation in a CSTR with a partial condenser is presented. The simulation code was developed in FORTRAN. Steady-state modelling leads to a system of both non-linear algebraic and second-order non-linear differential equations. Solution of the system is provided by the imported solver BUNLSI. Within every iteration a boundary value problem (reaction and mass transfer in the liquid phase) had to be solved with a solver from the IMSL mathematical library. The influence of the Hatta number on the CSTR performance was investigated for a simple second-order reversible reaction. (C) 2007 Elsevier B.V. All rights reserved.