This paper presents the use of vision-based measurement techniques for the on-line monitoring and characterisation of coal-biomass co-firing flames on an industrial-scale combustion test facility. The characteristic parameters of the flame, including ignition points, brightness, temperature and oscillation frequency, are measured using a vision-based flame monitoring system in an on-line continuous mode. A common type of pulverised coal was fired with five different types of biomass for different proportions ranging from 0% to 20% by weight. Various combustion conditions were created during the test runs, including variations in excess air and the method of biomass injection. The relationships between the flame characteristics and the primary combustion process data are analysed. The results suggest that, due to the varying physical and chemical properties of the biomass fuels, the biomass additions impact on the characteristics of the flame, particularly the flame ignition points and brightness. However, in all cases studied, the flame stability has been found to be little affected by the amount of biomass added provided that the addition is no more than 20%. (c) 2007 Elsevier Ltd. All rights reserved.