Wet ball milling, as a potential means to decrease the particle size of zeolite HY with minimal loss of crystallinity, was investigated. The diameter of the ball as well as the milling speed and time were varied in the experiments. Particle size distributions of the commercial zeolite HY and the ground samples were obtained to determine the variations occurring in the particle size and size span when wet ball milling was applied. XRD analyses, adsorption experiments and thermogravimetric analyses were performed for the characterization of the samples. After 2 h of wet ball milling, the medians of the particle size distribution curves by volume and by number could be reduced from about 6 and 2 mum to about 1 mum and 70 nm, respectively, accompanied by a crystallinity loss of about 10%, as determined from XRD analyses. The crystallinity decreased by about 45% for the longest milling time investigated, which also resulted in a relatively small particle size. The faster milling speed led to smaller particles with wider size distributions while the crystallinity was hardly affected. The utilization of the smaller ball diameter resulted in slightly smaller particles, only after relatively longer milling times, with narrower size distributions. Wet ball milling seemed to result in significantly higher crystallinities of the zeolite HY samples when compared to the results reported in the literature for ball milling performed under dry conditions. (C) 2004 Elsevier B.V. All rights reserved.