Two methods for determining mean particle size of an ultrafine particle size distribution were compared using a combustion aerosol source. The first utilized the scanning mobility particle sizer (SMPS), while the second combined measurements from the electrical aerosol detector (EAD), which measures total aerosol length (mm/cm(3)), and the condensation particle counter (CPC). Poor agreement was found between EAD/CPC and SMPS data when mean particle size was used as a basis for comparison but good agreement was found between EAD and SMPS data when total aerosol length was used as a basis for comparison. The largest contribution to the poor correlation between the two methods was the difference in concentration measured by the SMPS and the CPC. This paper examines the influence of both diffusion losses and nanoparticle aggregate mobility on these results, which also suggest that morphology, charging and composition may be contributing factors. (C) 2007 Elsevier Ltd. All rights reserved.