Aerosol particles present in cirrus crystals sampled with a CVI (Counterflow Virtual Impactor) have been analyzed for their thermal stability. The observations were conducted within the international project INCA (Interhemispheric differences in cirrus properties from anthropogenic emissions). The sample air carrying residual particles from evaporated crystals are divided into three branches. One branch is held at approximately 25C, the second is heated to 125C and the third is heated to 250C. After heating, all branches are held at approximately 25C before the residual particles are counted by three different condensation particle counters (CPC’s). The fraction of particles that disappear between 25C and 125C are termed volatile particles. Particles that disappear between 125C and 250C are termed semi-volatile, and particles that remain after heating to 250C are termed non-volatile. The relative abundance of these three different types of particles is compared between measurements conducted in the Northern and Southern Hemisphere midltitudes, respectively. The data reveal that cirrus clouds in the Northern Hemisphere midlatitudes contain a larger fraction of volatile particles (50 to 70%) by number compared to the Southern Hemisphere (40 to 60%).The non-volatile fraction often exceeded 30% and sometimes reached more than 50%. The contribution of semi-volatile particles was very small and essentially significant only at high crystal number densities in the Southern Hemisphere where it sometimes exceeded 10 %. The difference in the relative fractions of volatile, semi-volatile and non-volatile particles are surprisingly small when comparing data collected above and below 235K within one campaign. However, the larger difference between the two campaigns may reflect the different chemical properties of the ambient aerosol.