The comparison of particle mass spectrometry (PMS) and transmission electron microscopy (TEM) as size determining techniques has been performed on nanosized particles produced in two different types of low-pressure gas-phase reactors. Results from PMS measurements are compared to image analysis carried out on particles in situ sampled on TEM grids either by non-intrusive molecular beam probing or by a high-speed sampling method. It has been shown that for flame generated Fe2O3 nanoparticles the agreement between PMS measurement and TEM imaging is very good. ZnO particles, synthesized in a microwave plasma reactor show a similar good agreement for the respective analysis methods. To the contrary, the measured size of GeO2 particles originating from a plasma process differs for the methods employed. PMS measurements yield smaller particles compared to TEM image analysis of molecular beam sampled particles which can be attributed to uncertainties in the physical properties of nano-GeO2 particles. (C) 2002 Elsevier Science Ltd. All rights reserved.