A limit on particle number emission from the exhaust of Euro 5/6 engines will take effect in 2011 and engine exhaust particle number concentrations are required to be measured by particle number counters that meet requirements set forth by the UN/ECE particulate measurement programme (PMP). The TSI Model 3790 engine exhaust condensation particle counter (EECPC) was developed to meet these PMP requirements. This paper describes an experimental study that evaluated the performance of the EECPC. Experimental setup and procedures for calibrating EECPCs were evaluated. It was found that the EECPC counting efficiency strongly depended on the aerosol material. Among several materials tested, the counting efficiencies were highest for poly-a-olefin and lowest for sodium chloride. Errors in the concentration measurement due to the aerosol material dependence were estimated and found to be significant. Considerations for selecting aerosol materials for EECPC calibration are discussed with regard to best match with engine testing conditions. (c) 2010 Elsevier Ltd. All rights reserved.