Possible reaction mechanisms responsible for the release of Na and Mg during pyrolysis at elevated pressures are described in this paper. In order to evaluate these mechanisms a Victorian brown coal, Loy Yang coal, was pyrolysed in a wire-mesh reactor at pressures up to 6.1 MPa at a heating rate of 1000 degreesC s(-1). Release of Na and Mg were quantified as functions of temperature and pressure. The results demonstrated that increasing pressure suppresses or promotes release of Na and Mg depending on the combination of pressure and temperature. The results obtained have been explained qualitatively by the proposed reaction mechanisms. At temperatures of 600 degreesC and lower, the release of Na and Mg from the pyrolysing coal/char particles, as light carboxylates, other organic salts and/or metals, was controlled by their diffusion through the pore system of the particles and, therefore, was suppressed by increasing pressure. At higher temperatures, the release of Na and Mg seems to be affected by the changes in intra-particle mass transfer mechanism due to increasing pressure as well as by chemical reactions responsible for the formation of volatile Na and Mg species. (C) 2003 Elsevier Science Ltd. All rights reserved.