Four new, four re-calibrated and six used TSI 3790 condensation particle counters (CPC) were checked using soot particles produced by a mini CAST generator. One of the re-calibrated CPC was also checked with Emery oil (PAO 4) particles (evaporation-condensation method) and its counting efficiencies were in agreement with the manufacturer's calibration data which were determined with Emery oil particles (electrospray method). However, the counting efficiencies with the soot particles were lower indicating the importance of the calibration material (chemical composition, morphology). Since UN-ECE regulation 83 for light duty vehicles does not specify the material to be used, the validation (check) should be conducted with the same material used by the manufacturer or the differences between the materials should be taken into account. Two used CPCs showed a similar to 20% decrease in their counting efficiencies after 1.5 years of measuring engine exhaust aerosol. This drift could not be recognized by the status indicators of the instrument and could be attributed to a degradation of the CPC saturator (wick). Two CPCs also showed a non-linear response: a 7% difference was found between low (10 cm(-3)) and high concentrations (10,000 cm(-3)). A non-linear response (up to 40%) was also observed when the temperature difference between the saturator and the condenser was increased to decrease the cut-off size of the CPCs. (C) 2011 Elsevier Ltd. All rights reserved.