ddDevelopment of more effective technologies of utilising low rank coals for power generation has been driven by a demand for higher efficiency, low capital costs and minimal environmental impacts. Fluidised bed systems are regarded as one of the more promising alternative technologies for power generation to overcome the disadvantage of the existing pulverised coal burning power generation plants for low rank coals. However, ash deposition and bed agglomeration are potential problems in fluidised bed processing of coals with high alkali and sulphur contents. In order to gain a better understanding of the mechanism of agglomeration in fluidised beds, a good knowledge of the rheological behaviour of coal ash deposits at high temperatures and under the processing conditions is necessary. Rheological characterisation of materials at high temperatures is difficult due to lack of standard instruments and reliable measurement techniques. We have recently developed a rheometer that has the capability of measuring the theological properties of coal ash slag over a wide range of temperatures from 600 to 1300degreesC and under different processing atmospheres. In this paper the features of this unique instrument are described and the experimental technique developed for flow property measurement is outlined. Some typical measured rheological properties of coal ashes from different Australian low-rank coals are presented and discussed to illustrate the potential applicability of the rheometer for high-temperature theological characterisation. Furthermore, by means of the experimental results obtained it is demonstrated that the alkali content of the coal ash plays a significant role in controlling the rheological characteristics of the ash deposit, which in turn has an important implication on agglomeration in fluidised bed combustion processes.