Design and optimization of evaporators and condensers requires a reliable simulation of the change of states at the air and refrigerant side. Heat transfer and pressure drop in two phase flow systems with HFC-134a/oil mixtures are strongly dependent on local parameters. To take into account oil effects, it is necessary to modify the correlations for heat transfer and pressure drop which are proposed in the literature for pure refrigerants. At the air side of evaporators the local heat transfer coefficient and heat transfer rate are dependent on whether the local surface temperature is below the dew point of the humid air or not. If partial vapor condensation occurs, mass transport will affect the sensible heat transfer coefficient and fin efficiency. In order to simulate the thermophysical behavior of evaporators and condensers, it is necessary to evaluate local values for heat and mass transfer at both sides and therefore to devide the evaporator and condenser into a number of volume elements (cells).To validate the simulation results with the cell model, extensive comparisons with experimental results have been carried out and satisfactory agreement has been found.