This item was submitted to Loughborough's Institutional Repository (https://dspace.lboro.ac.uk/) by the author and is made available under the following Creative Commons Licence conditions.For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/ ABSTRACTNew European emissions legislation (Euro5) specifies a limit for Particle Number (PN) emissions and therefore drives measurement of PN during vehicle development and homologation. Concurrently, the use of biofuel is increasing in the marketplace, and Euro5 specifies that reference fuel must contain a bio-derived portion.Work was carried out to test the effect of fuels containing different levels of Fatty Acid Methyl Ester (FAME) on particle number, size, mass and composition. Measurements were conducted with a Cambustion Differential Mobility Spectrometer (DMS) to time-resolve sub-micron particles (5-1000nm), and a Horiba Solid Particle Counting System (SPCS) providing PN data from a Euro5-compliant measurement system. To ensure the findings are relevant to the modern automotive business, testing was carried out on a Euro4 compliant passenger car fitted with a high-pressure common-rail diesel engine and using standard homologation procedures.It was found that using FAME decreased total PN emissions, by 16% over the Type I drive cycle (NEDC) for a 30% biodiesel (B30) compared to mineral fuel (B0). FAME also decreased accumulation mode PN and carbonaceous mass, by 20-30% for B30 versus B0, with a consequent reduction in Diesel Particulate Filter loading rate. A 25% increase in the nucleation mode PN was found when using B30 versus B0, and the higher molecular-weight organic mass fraction also increased. Increases in nitrogen oxides when using FAMEcontaining fuels were also confirmed.