This paper discusses issues which will appear if the PMP methodology is extended to the measurement of particles with sizes below 23 nm. In addition, the origins of sub 23 nm particles from three different types of engines are discussed. An increased uncertainty was observed for the detection efficiency measurements with a particle number counter (PNC) at particle sizes below 10 nm. The particle concentration reduction factor (PCRF) of a volatile particle remover for 15 nm particles was higher than those for 30, 50 and 100 nm particles because of increasing particle losses. Then, exhaust particle emissions from a gasoline direct injection (Dl) passenger car and trucks with and without a diesel particulate filter (DPF) were measured using the PMP methodology with a PNC whose D-50 value was 2.5 nm (PNC_2.5) and a normal PMP PNC (PNC_23). The fluctuations of the results for PNC_23 and PNC_2.5 were almost the same when the effect of re nucleation of semi volatile particles was removed. Different procedures for estimating the PCRF showed varying results, -23% to +41% in the worst case. The profiles of sub-23 nm particles from diesel trucks were similar to those of particles larger than 23 nm. In the case of the gasoline DI car, a peculiar emission Corresponding to hydrocarbon nucleation was observed in high-speed operation. Particles emitted from the diesel engines originated from combustion processes in the case of the truck without the DPF and were strongly affected by the DPF conditions in the case of the truck with the DPF. (C) 2015 Elsevier Ltd. All rights reserved.