The infrared emission spectra of a variety of small diffusion flames was studied in the laboratory. Ethylene, acetylene, matchwood and the pyrolysis products of Eucalypt forest litter gave quantitively similar spectra consisting largely of non-specific black body radiation from carbon particles, with narrow radiation bands due to water vapour and, especially, carbon dioxide. Addition of water to the fuels had little effect on the carbon dioxide emission but reduced the radiation from the carbon particles by a factor as large as three. The effect arises from changes in the opacity of the flames. This result goes far towards explaining the profound influence which fuel moisture has on the rate of spread of a forest fire.