Kinetics of the dissolution of particulate polycrystalline MgO in excess of hydrochloric acid were investigated, with special regard to the effect of activity of H+ ions, temperature and particle size. The results are presented in terms of the shrinking particle model. The overall process is controlled by the chemical reaction of MgO with H+ ions at the liquid-solid interface. The apparent activation energy was 58 kJ mol(-1). Acid concentration (from 0.5 M to 5.1 M) was surprisingly found to have a decelerating effect on the dissolution rate of MgO, resulting in negative values of reaction order for H+ ions (from -0.16 to -0.18). We conclude that the regime studied here, characterised by decrease in the dissolution rate of MgO with an increase in H+ activity, occurs under far from mass-transfer control conditions in the solutions with low Mg2+:H+ molar ratio. (c) 2006 Elsevier B.V. All rights reserved.