Depolarization data of stratospheric aerosols obtained by ground-based polarization lidar are analyzed. The measurements were performed at Koldewey station, Ny-¿lesund, Spitsbergen (78.9 ¿N, 11.9 ¿E), during the five winters of 1994--1995 to 1998--1999. The study is focused on stratospheric sulfuric acid aerosols; interference by nitric acid trihydrate containing particles is excluded by imposing a temperature threshold condition of 2 K above the nitric acid trihydrate coexistence temperature. Under the assumption that large sulfuric acid particles freeze first, T-matrix scattering calculations show that volume depolarizations start to increase significantly if ~1% of the aerosols are frozen. Observations in January 1999 and February 1998 indicate the presence of nonspherical particles. We interpret these observations as signatures for the occurrence of sulfuric acid tetrahydrate particles. Temperature histories obtained from back trajectory model calculations and the corresponding polarization lidar measurements are consistent with current knowledge of sulfuric acid tetrahydrate formation. ¿ 2001 American Geophysical Union