Heteroepitaxial thin film ZnS and ZnSe layers were grown on (100)-oriented GaAs substrates in the temperature range from 290 to 420 degrees C with a H-2 dilutant. The growth studies were performed in a conventional vertical metallorganic vapor-phase reactor with a downstream inclined substrate holder. Numerical calculations were carried out to optimize growth rate homogeneity, flow patterns, and the input partial pressures of the limiting growth precursors. The surface morphology, the average surface roughness, and layer thickness distribution were evaluated by scanning electron microscopy and by profilometry analyses. The experimental results closely match the numerical data. Furthermore, the quality of layers were characterized by X-ray diffraction and photoluminescence methods to underline the correlation to the flow behavior of the gas mixture.