During the EU Project PartEmis, the microphysical properties of aircraft combustion aerosol were investigated. This study is focused on the ability of exhaust aerosols to act as cloud condensation nuclei (CCN). The combustor was operated at two different conditions representing old and modern aircraft engine technology. CCN concentrations were measured with the University of Vienna CCN counter <Giebl et al., 2002> at supersaturations around 0.7%. The activation ratio (fraction of CCN in total aerosol) depended on the fuel sulphur content (FSC) and also on the operation conditions. CCN/CN ratios increased from 0.93 through 1.43 to 5.15 ¿ 10-3 (old cruise conditions) and 0.67 through 3.04 to 7.94 ¿ 10-3 (modern cruise conditions) when FSC increased from 50 through 410 to1270 ¿g/g. The activation behaviour was modelled using classical theories and with a semi-empirical model <Gysel et al., 2003> based on measured hygroscopicity of the aerosol under subsaturated conditions, which gave the best agreement.