Polar stratospheric clouds (PSCs) were observed on several flights during the 1999/2000 SOLVE/THESEO mission. Here we present an analysis of PSC size distribution, composition, and particle phase based on near coincident measurements using the Multiangle Aerosol Spectrometer Probe (MASP) and Focused Cavity Aerosol Spectrometer (FCAS III) on board the ER-2 as well as lidar instruments on board the DC-8 and Falcon aircrafts. We calculate the aerosol backscatter ratios and aerosol depolarization ratios at infrared and visible wavelengths based on the particle size distributions measured by the MASP and FCAS III instruments. We then compare our calculations to observed lidar measurements taken from on board the DC-8 and Falcon aircraft, which flew the same flight paths as the ER-2 on 20 January and 3 February 2000, respectively. Our comparison shows that calculations based on the mixed clouds containing small (submicrometer) spherical particles and large (2.5--22 ¿m diameter) nonspherical particles seen by the FCAS III and MASP are consistent with lidar observations. Our analysis shows that the infrared aerosol depolarization ratio must be measured to detect and identify mixed PSC clouds and that relying only on measurements of depolarization in the visible causes incorrect identification of mixed clouds as solution droplets. This study suggests that mixed clouds of supercooled ternary solutions were observed in coexistence with larger solid nitric acid hydrate particles.