The influence of aerosol flow rate and temperature difference between saturator and condenser on the condensation particle counters (CPC) 3762 counting efficiency was investigated. Experiments were carried out for two different aerosol flow rates (1.5 and 3.0 l/min) and four temperature differences (15, 17, 20 and 23 °C) between saturator and condenser. The resulting particle counting efficiency curves and 50% particle detection diameters are presented.