The influence of particle aggregates on particle sizing with optical particle counters (OPC) is demonstrated. First, the scattered light intensity for a specific OPC is calculated for clusters of 2, 3 and 4 spheres in the size range of 0.01-2 mum. The intensity curves are compared with those of spheres with equivalent volume diameter in the same size range. The calculations were validated experimentally for aggregates of spherical polystyrene and silica particles. The enhancement of scattered light intensity due to irregular particle shape is demonstrated by the comparison of experimental calibrations with the above calculations for clusters. (C) 2000 Elsevier Science Ltd. All rights reserved.