Indian high-ash coal contains a-silica components of the mineral matter. When coal is transported in the form of a slurry, a-silica adds to the settling properties of coal and enhances erosion of pipelines. As Such any technique that will contribute to changing the characteristics of mineral matter by selective demineralization is bound to Supplement the knowledge of coal slurries in the science of rheology. One such method is the use of a novel microwave technology, which changes the a-silica to less harmful beta-silica. Thus microwave-treated coal slurry facilitates enhanced flow characteristics and abates the erosion problem in pipeline transport as well as in coal-slurry injection furnaces. This paper reports on the rheological study of closely sized coal particles of medium-volatile, low-ash, low-moisture cleans and high-ash rejects with and without microwave treatment. Viscosity of suspensions of microwave-treated coal was found to be less than that of untreated coal, in the case of both cleans and rejects. Microwave pretreatment thus reduces the viscosity and the Pumping cost and opens a new outlook for pipeline transport. An attempt has been made to quantify the improvement of rheological characteristics due to microwave pretreatment. (C) 2004 Elsevier Inc. All rights reserved.