An experimental test rig for the study of impact in an air jet mill (up to 500 m s(-1)) allows the evaluation of the relationship between the impact energy and the fineness of the impacted solids. In single impact tests, so far, three types of behaviour are found: brittle (for glass, sand, polyamide and NaCl), "complex" (for Al(OH)(3)) and ductile (for PMMA). A classification is obtained from the definition of grindability parameters. A multi-impact study reveals the non-equivalence between multiple impact and single impact: a solid is not broken similarly if a given total energy is provided in one or several impacts. (C) 2003 Elsevier Science B.V. All rights reserved.