The sizing accuracies of two widely used yet hitherto unevaluated optical particle counters (OPCs-Grimm Model 1.109 and Palas Model WELAS 2100) as well as one high-resolution. non-commercial OPC were evaluated. The measured data were compared to scattering intensity calculations based on Mie theory. Additionally, the counting efficiency for all three counters was measured, as was the influence of coincidence effects for the OPC with the lowest (manufacturer specified) upper concentration limit. Beside the traditional polystyrene latex calibration, a little-known, very fast and precise "multimodal" calibration method was used, which is based on the simultaneous generation of up to eight sharp multiple-charge modes from polydisperse di-ethyl-hexyl-sebacate (DEHS) particles by electrical mobility classification. (C) 2008 Elsevier Ltd. All rights reserved.