Precision metrology corrosion probes have been exposed in a 1MW(th) combustion test facility to investigate the influence of operational and fuel variables on the furnace wall fireside corrosion rates of power station boiler tubing. Negligible corrosion occurred under predominantly oxidising local flue gas conditions. The corrosion rates were found to be dependent only upon metal temperature, with no effects of coal composition or heat flux being discernible. Under reducing conditions, the corrosion mechanism shifted progressively from oxidation towards sulphidation as the CO content of the local flue gas increased. There was no measurable effect of heat flux on the corrosion rates in the absence of chlorine, but increasing the CO content of the local flue gas increased the corrosion rates via the formation of progressively more sulphide within the corrosion scale. In the presence of chlorine, metal chlorination, via the HCl/FeCl2 mechanism, came into play and dominated the overall metal loss. Linear reaction kinetics ensued and these were strongly influenced by a close interrelationship between coal chlorine content and heat flux.