Biodiesel use in the near future is expected to grow considerably, since it is renewable, has lower net CO2 emissions, and due to the effect of legislation. All diesel fuels produce harmful emissions. Particulate matter (PM) is one of the major concerns. PM has a detrimental impact on the environment and human health, with small nano-sized particulates being especially hazardous. It is worthwhile to determine the nature of PM produced from diesel fuels, especially those derived from biodiesel, and also to ascertain whether current technology is suitable for PM exhaust removal for biodiesel. In this study ULS diesel and biodiesel (rapeseed methyl ester (RME)) were tested for combustion and emissions in a single cylinder research engine with no aftertreatment and then with aftertreatment (diesel oxidation catalyst (DOC) and diesel particulate filter (DPF)). A host of online and offline PM analytical techniques were used. It was found that RME combustion produces low emissions of unburnt total hydrocarbons (THCs), carbon monoxide (CO) and particulate matter (PM), but has increased emissions of NOx. Dangerous unregulated carcinogenic compounds, acetaldehyde, formaldehyde, benzene, and 1,3-butadiene, were found in ULS diesel exhaust and in reduced quantities for RME. RME combustion produces significantly less solid PM (less than half) but slightly more liquid PM compared to ULS diesel. RME PM is decreased at nearly all PM sizes. However, at the small nano-particulate size RME combustion produces an increased proportion of nano-particulate and these were revealed to be of soluble organic fraction (SOF) nature (liquid PM). More than 99% of the solid PM by mass and number, was trapped by the DPF for both ULS diesel and RME. Liquid PM was filtered by more than 88% and 80% for ULS diesel and RME respectively on a mass basis (96%, 95% number basis). The bulk of the remainder is thought to be water PM. At all times overall PM levels for RME were lower than ULS diesel. (C) 2012 Elsevier Ltd. All rights reserved.