A programme of particle measurements at two different direct injection (DI) diesel engines have been performed in which the effect of H2O2 addition to the exhaust gas on particle emissions has been observed. A DDMA-system consisting of two synchronously operating DMA-CPC systems one bring equipped with a thermolysis tube was used to separate the effect of H2O2 addition on the solid and on the volatile particle material. The results indicate that the effect on solid particle material is very weak due to a lack of reaction time. This result is confirmed by Bosch Smoke Number measurements. On the other hand, a clear effect of H2O2 addition on the volatile particle fraction was observed. Additionally performed tandem DMA measurements gave hints to the kind of volatile species. (C) 2000 Elsevier Science Ltd. All rights reserved.