The cross piston press possesses four hydraulic cylinders mounted crosslike. Each of them acts on one piston of the working block in the center. The oppositely orientated pistons are activated simultaneously and the both piston pairs act alternatively. In this way a multiple crosswise stressing of the material in the working chamber of the block is achieved. The cross piston press can comminute a fine material to a microfine powder. Locked-cycle experiments were done with limestone. quartz and smelted corundum, and a product fineness was achieved of x(95)=3-4 mum. The specific energy consumptions were 60 kW h/t with limestone, 300 kW h/t with quartz, and 480 kW h/t with corundum. these values are remarkable less then those ones of milling in stirred mills. Considerations on the throughput lead to the results that industrial units could have a capacity up to 4000 kg/h (limestone), 1500 kg/h (quartz), and 400 kg/h (corundum). The test device was designed for a dry operation, however it is possible after some modifications to perform a wet comminution with a high concentrated slum. A cross piston press might also be employable for mechanical activation. (C) 2004 Elsevier B.V. All rights reserved.