Nanoparticle size distributions and their concentrations were Studied in atmospheric premixed ethylene/air flames using photo ionization mass spectrometry (PIMS) and total organic carbon (TOC) calibration Supplemented by differential mobility analysis (DMA). Focus Of this Study is the evolution of nanoparticles as a function of height above burner (HAB) and of the C/O ratio of the unburned gases. It was found that especially particles of the cluster type exhibit a sharp concentration drop by more than two orders of magnitude within a narrow C/O window which is close to the sooting threshold. Using DMA a decline by two orders of magnitude was found. These results Suggest that at best only small concentrations of nanoparticles should be formed significantly below the sooting threshold. As these conditions prevail in a homogeneously charged IC engine no or only very small nanoparticle emissions are expected in the exhaust gas. This was indeed found for a small Otto engine driving a power generator unit. Using flame nanoparticle profiles as standard, absolute concert I rations for their emissions could be deduced. These data were supported by additional DMA measurements. The Calibration using TOC did not completely match the one based on the condensation particle counter of the DMA apparatus. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.