The influence of physical properties and the resulting limitations of heat and mass transport for the ethoxylation of octanol in a microstructured reactor were investigated by CFD simulations. The reaction was performed under supercritical conditions in a one phase system at pressures between 90 and 100 bar and temperatures between 180 and 240 degrees C. A 2D CFD model was applied to determine the kinetic parameters by fitting the model to experimental data. Furthermore, the sensitivity of the simulations regarding different physical properties was determined. The influence of diffusion was studied in detail, and it was found that a low diffusion coefficient results in a radial gradient in viscosity which leads to segregated streamlines and a bending of the streamlines toward the center. This "bottleneck" effect causes a large increase of the velocity in the center of the pipe, with the risk of the breakthrough of ethylene oxide. This effect occurs only in the case of higher ethoxylation degrees.