Many previous studies have investigated the influence of microwave pre-treatment upon coal grindability. In most cases these studies showed that significant increases in coal grindability could be achieved after periods of exposure to microwave energy. Unfortunately, few of these workers considered the effect of the microwave treatment upon the chemical or more importantly the petrographic properties of the coal. Although it is already clear that microwave pre-treatment of coals increases their grindability, it is vital from a process point of view to understand how the coals are altered in the process, with respect to burnout and potential burnout problems. Five different coals from around the world were ground and sieved into a +1.18-3.00 mm size fraction and then treated with high electric field strength microwave energy for 0.1 s at four different power settings (3, 7.5, 10 and 15 kW). 200-300 g of each coal was treated in a Te010 single mode cavity using a pneumatic system to control the residence time of each sample in the cavity. This study looks at how the coals fundamental properties change as a result of exposure to relatively high levels of microwave energy. The properties investigated include moisture content, fuel ratio petrography and intrinsic reactivity as determined by a thermogravimetric analyser. (C) 2004 Elsevier Ltd. All rights reserved.