The kinetics of the primary particle growth and the restructuring of nanoparticle agglomerates of Ni and Pt were studied under variation of temperature and gas composition. The aerosol particles used in the study were produced by spark discharge into nitrogen as carrier gas. Restructuring was monitored by measuring the mobility equivalent diameter by SNIPS after different residence times in a tempered volume. To determine the kinetics of primary particle growth, samples were taken over a range of residence times for TEM analysis. These experiments were conducted in reducing atmospheres and in pure nitrogen to study the effect of surface state on the kinetics of the processes. To quantify sintering kinetics, a multi-stage sintering model based on the reduction of the surface energy was fitted to the experimental results. A strong dependence of the primary particle growth and the agglomerate restructuring on the surface state of the particles was found. Both processes were accelerated strongly with increasing purity of the particle surfaces. The model yielded an activation energy for the primary particle growth of Ni-agglomerates in pure nitrogen (99.99%) of about 80 kJ/mol which was decreased for reduced Ni and Pt particles to a value of about 50 kJ/mol. The kinetics of restructuring was much faster than the one for primary particle growth. This enables the seperation of the manipulations of agglomerate structure and primary particle size. (C) 2003 Elsevier Ltd. All rights reserved.