As a result of increased concern over the toxicological effects of particulate matter (PM) emitted by compression ignition vehicles, the European Union (EU) has introduced a particle number (PN) limit. This represents a significant departure from the traditional gravimetric approach of quantifying PM emissions, and introduces unique challenges to the automotive testing process. The legislation sets certain limits and guidelines for systems designed to quantify particle number emissions, but the legislation currently has some flexibility regarding system layout, operating temperatures, etc. In this work, two commercially available particle counting systems were tested with a variety of Euro 5 light-duty CI vehicles equipped with particulate filters (DPFs). The results indicate a small but reasonably consistent discrepancy between the two systems, with a mean difference of 9.3%. Possible causes of this difference (which was not observed in all cases) are discussed in the context of the current EU light-duty CI PN limit, and possibilities for future research directions are suggested.