For the collection of diesel soot particles, a two-stage collection system is employed consisting of a cyclone in combination with a preceding electrostatic agglomerator. By virtue of its design, the agglomerator is, in principle, a tubular electrostatic precipitator. The agglomeration mechanism is based on the fact that the electrically conductive soot particles accumulate at the collecting electrode of the agglomerator as flake-like structures. These are re-entrained when they exceed a certain size. The process of agglomeration can therefore be divided into three steps, i.e. particle transport, formation of agglomerates and emission. In order to assess the efficiency of the agglomerator, a measuring methodology has been developed which allows the determination of the size distribution of the soot particles over the range 17 nm to 40 mum. Investigations carried out in connection with this work show that the proportion of particles bound in agglomerates depends essentially upon the operating level of the engine. Depending on the various conditions employed, more than 90% of the particle mass may be bound in agglomerates whose sizes exceed 3 mum. With increasing engine load, the proportion of agglomerates decreases to ca. 40%. Varying the gas volume flow at a constant engine operating level demonstrates that such a dependency on load is not related to residence time, i.e. low efficiency does not arise from insufficient particle transport, but appears to be associated with one of the subsequent stages of the agglomeration process. Details of these processes have not been investigated so far. The total collection efficiency achieved via this system varies between 43% and 87% according to the various operating states of the engine. This significant difference can be explained in terms of the dependence of agglomeration and agglomerate collection upon engine load and speed.