Defined particle bed arrangements are an appropriate and useful tool for studying the substantial comminution behaviour of a particle collective. Two fractions of the material limestone were comminuted batch-wise by high pressure in a die press under defined conditions for eliminating the wall effects of the confinements. The particle interactions were modified by using the additive caproic acid and by varying the lateral confinements. The influence of feed size, additive amount and confinement on the specific surface growth and the energy absorption was investigated. It could be proved that a decrease of internal friction due to the additive exerts a small effect on the comminution process, whereas the material transport out of the stressing zone is decisive. For higher amounts of the additive, a decrease of the energy absorption in the fully confined particle bed with a concurrent decrease of the specific surface growth could be shown. On the contrary, there is a very high increase of the energy absorption in the partially open particle bed arrangement according to a high mass transport and specific surface growth. The inhomogeneous densification was observed in a half-cut configuration with a CCD camera. Superpositioning the pictures from tests with vertical and horizontal layers enables to study the reached densities in a grid pattern. (C) 2004 Elsevier B.V. All rights reserved.