In simulation models, used to describe the formation of Polar Stratospheric Clouds (PSC), it is usually assumed that the background population of sulfate aerosols acts as nucleation centers for the PSCs. At normal stratospheric temperatures the aerosols are in the liquid phase, not suitable for PSC nucleation, but still it has been assumed that the particles will freeze above the nitric acid trihydrate (NAT) condensation temperature. Homogeneous freezing of aerosol particles into sulfuric acid tetrahydrate and ice has been incorporated in a detailed microphysical PSC-model to investigate the impact of freezing among the sulfate aerosols on the PSC size distributions. It is demonstrated here that freezing of the largest particles might explain the observed high HNO3/NAT saturation ratios and denitrification by particle sedimentation in the Arctic stratosphere. Comparisons have also been made to the PSC formation under Antarctic temperature conditions.