The hydrothermal dewatering of a Loy Yang Low Ash Victorian brown coal has been studied under a wide range of process conditions. The intra-particle porosity (< 1 mum pore radius) determined by mercury porosimetry of the resulting dried products, a proxy for the maximum solids concentration of coal-water slurries, is reported as a function of reaction time, processing temperature, autoclave size and configuration and slurry concentration. Of these variables only the process temperature had a significant influence on the product intra-particle porosity, which decreased with increasing temperature. Other product variables were affected by a wider range of process conditions; increase in temperature and residence time and decrease in slurry concentration led to marked increase in loss of organic material to waste water, and the elemental composition of the product changed significantly with e.g. residence time. Drying of coal samples of particle sizes > 50 mum gave products of constant intra-particle porosity but smaller coal particles gave products with higher intra-particle porosities as a result of agglomeration. Milling of the coal reduced the porosity of the dried coal at constant particle size. (C) 2002 Elsevier Science Ltd. All rights reserved.