We investigated the possibility of extending the legislated particle number (PN) measurement procedure to (a) control diesel exhaust emissions under unregulated driving conditions, including tests at -7 degrees C and during regeneration of the particulate filter, and (b) reduce the lowest detectable size below 23 nm, through dedicated tests of a Euro 5b and a Euro 6 diesel vehicles. The PN emissions of the two vehicles were at or below the limit of 6 x 10(11) #/km under all non-regenerative conditions examined. Active regeneration of the Diesel Particulate Filters (DPFs) occurred more frequently (220-510 km) than previously reported, and resulted in elevated PM (up to 11 mg/km) and PN (1-4 x 10(12) #/km) emissions. Condensation Particle Counters (CPCs) having a nominal 50% detection efficiency (10 nm and 4.5 nm) below the legislated 23 nm, detected 50-60% more non-volatile particles over the certification test cycle (New European Driving Cycle). Even higher excess fractions of nano-sized non-volatile particles (160-500%) were observed under urban driving of the Euro 5b vehicle at - 7 degrees C, with the absolute levels still remaining below 6 x 10(11) #/km however. DPF regeneration and motorway driving favoured the homogeneous nucleation of volatile particles in the Constant Volume Sampler (CVS), which also contributed to the signals of low cut-off size CPCs sampling downstream of the Volatile Particle Remover (VPR). No evidence of such volatile interference could be observed in the legislated PN procedure. A potential shift of the lowest detectable size towards smaller sizes, will require further developments in the measurement procedure. (C) 2012 Published by Elsevier Ltd.