Nucleation mode particles were investigated for their morphology using TEM and the presence or absence of solid cores was addressed. At cold start idle nucleation particles were observed in the exhaust of a diesel passenger car. These particles occurred with both low and high S fuel and were only partly volatile in a thermodenuder, which indicates that the composition was not sulfate and as derived from TEM/EDX (transmission electron microscopy/energy dispersive X-ray analysis) probably not ash. it could be high boiling hydrocarbons, or primary soot particles. With all fuels at warm idle no nucleation particles and only soot particles were observed in the SMPS and the TEM. With 3.0x10(11) s(-1) the total soot particle number during idle was much less than during driving, e.g. at 120 km h(-1) the emission rate was 6.7x10(12) s(-1). At high load and high S fuel 10-20 nm nucleation particles were observed by SMPS and TEM. A thermodenuder at 280 degrees C and TEM showed that all nucleation particles were volatile. EDX gave a weak S-signal only. Some nucleation particles contained smaller spots (1-3 nm) with a very high contrast, which might be due to heavy elements. However, under the electron beam of the TEM these spots disappeared and EDX analysis was not possible. With low S fuel at 120 km h(-1) only soot particles and no nucleation particles were observed. (C) 2008 Elsevier Ltd. All rights reserved.