The influence of atomising gas pressure on particle shape of inert gas atomised Al and Mg powders was investigated in a pilot plant gas atomiser using an IN4/ON18/3.5-4.0 type 'confined design' nozzle. In the tests for Mg powder production, argon at 1.05-1.85 MPa and helium at 1.03-1.77 MPa were used as the atomising agent. A controlled amount of oxygen in the gaseous phase was kept constant at 0.5% by volume while atomising gas pressure was changed from 1.06 to 1.85 MPa for argon runs and from 1.02 to 1.85 MPa for helium runs during Al powder production. The morphology of powder particles was examined by SEM. Powders were sized using dry sieving (under Ar for Mg powders) and wet sieving. The Sauter mean diameter varied from 12.97 to 24.13 mu m for Mg powders and from 9.2 to 20.7 pm for Al powders (dry sieve) depending on the atomising gas and its pressure. Size distribution curves indicated bi-modal distribution in the powder. The fine size range of the particles was spherical and the coarse size ranee exhibited particles with oblong and round shapes when the argon pressure was high during Mg powder production. Argon with low pressure caused production of completely spherical Mg powder particles with a tight size distribution. In helium runs of Mg powders, the coarse size range of the powder was more irregular with rough surfaces in high atomising gas pressure tests than that in low pressure ones. In Al powders, the same morphological features of powder particles were observed but the effect of the gas pressure on Al powder particle shape was lower. This is due to the more efficient secondary break up in the process of atomisation of Al compared with Mg. (C) 1999 Elsevier Science S.A. All rights reserved.