Presented are results of an inter-laboratory study (ILS) for measurements of the particle size distribution of fine powders in wet dispersion by laser diffraction. In this proficiency test 32 participants from four countries took part. They utilized 13 different devices from 7 manufacturers. Three commercial powders (glass spheres and two silicon carbide powders) showing a median diameter of about 30, 10 and 1?mu m (volume distribution), respectively, were chosen for the procedure. A homogeneity study was carried out after the units had been separated and bottled. All participants received their test samples including a description of the standard operating procedures based on ISO 13320:2009 to ensure that experiments were performed in a consistent manner. Results were calculated using the Mie Theory. The general means and the precision of the results were estimated in accordance with ISO 5725-2:2002. The evaluation showed excellent values of repeatability standard deviation. Values of 4 to 21?% of the reproducibility standard deviation of the results were found in the particle size range above 1?mu m. Much larger deviation between the labs was detected in the case of smaller particles. Differences in the design of the analyzers were unambiguously identified as the main reason for the large deviations.