The polarisation ratio resulting from the backscattering of linearly polarised incident light by fly ash particles is indicative of the carbon content. Determination of this parameter is useful for characterising the efficiency of coal burning furnaces. This paper describes the development of an instrument to make these measurements, which is designed to be attached furnace ducts. The performance of the instrument is tested in laboratory measurements on a range of ashes from different coals. It is concluded that if the mineral content of the ashes is not known then the carbon mass fraction can be determined to within +/- 1%. If the mineral content is known then the use of neural network analysis can reduce this to +/- 0.5%. (C) 2002 Elsevier Science Ltd. All rights reserved.