Packing characteristics of particle beds, determined mainly by the particle size distribution, exert a critical influence on the interparticle breakage of minerals in confined beds under compression. This paper is concerned with the effects of feed size distribution on characteristic phenomena of confined-bed comminution. Minus 3.35-mm quartz and calcite feeds were prepared by proportionately mixing previously classified narrow-size fractions so as to construct particle size distributions having the Gaudin-Schuhmann functional form with distribution modulus values of 0.5, 0.7, and 0.9. These feeds were comminuted in a piston-die press at five levels of applied pressure ranging between 26.5 MPa and 283 MPa, generating specific energy inputs in the range 0.65 J/g to 8 J/g depending on the material and feed size distribution. The size distributions of the comminuted solids were analyzed for self-similarity, comminution kinetics and reduction ratios. Force-displacement data were analyzed for bed compaction, energy absorption and energy utilization phenomena. The results showed definite effects of feed size distribution on these latter phenomena and product particle size distributions. The relationship between the specific energy absorbed and applied pressure is linear. Self-similarity does not hold for the coarse-size range of the products, indicating greater deviations for feeds with greater proportions of the coarse material. Breakage fraction of the top-size class (3.35 x 2.36 mm) particles and production of fines (minus 75 mu m) are significantly high with feeds having the size distribution modulus of 0.9. Reduction ratio-specific energy input relationship is curvi-linear; energy utilization becomes better with increasing value of the distribution modulus. The observed effects are explicable in terms of the packing density of particle beds and the amount of fines in the feeds. (C) 2009 Elsevier B.V. All rights reserved.