Measurement of particle number was introduced in the Euro 5/6 light duty vehicle emissions regulation. Due to the complex nature of combustion exhaust particles, and to transportation, transformation and deposition mechanisms, such type of measurement is particularly complex, and regression analysis is commonly used for the comparison of different measurement systems.This paper compares various commercial instruments, developing a correlation analysis focused on PN (Particle Number) measurement, and isolating the factors that mainly influence each measuring method. In particular, the experimental activity has been conducted to allow critical comparisons between measurement techniques that are imposed by current regulations and instruments that can be used also on the test cell. The paper presents the main results obtained by analyzing instruments based on different physical principles, and the effects of different sampling locations and different operating parameters.The main instruments that have been critically analyzed during this project are: Horiba MEXA 2000 SPCS Particle Counter installed on a CVS tunnel; AVL APC 489 installed directly on the exhaust gas flow; AVL Smart Sample 478 GEM 140 (Mini CVS tunnel) + AVL APC 489; Cambustion DMS 500 installed directly on the exhaust gas flow; AVL MicroSoot Sensor 483 installed directly on the exhaust gas flow. The tests have been carried out on a prototype vehicle equipped with a GDI engine, both under steady-state conditions and during the New European Driving Cycle (NEDC), while comparing the effects of different dilution factors, different engine calibration datasets, and different positions of the various instruments.