In order to evaluate the influence of the measuring technique on the determination of (micro-)aggregation in soil and sediment samples, results of grain size distributions of undispersed silty soil samples obtained by the sieve-pipette method are compared with those obtained using a laser diffraction grain size analyser, the Coulter LS-100. Reduced major axis relationships are calculated which may be used to convert Coulter LS-100 results to those obtained by the sieve-pipette method. The relationships obtained are very similar to the reduced major axis relationships established for dispersed silty soil samples. The results also show that the Coulter LS measurements have a systematic bias compared to the sieve-pipette data. This implies that, if the percentage of (micro-)aggregation is determined, the (interpretation of the) results will be strongly dependent on the measurement technique used. Using the calibration relationships that were established, nomographs can be developed to predict the level of sieve-pipette (micro-) aggregation from Coulter LS-100 data. Copyright © 1999 John Wiley & Sons, Ltd.