An experimental study of radiation heat transfer during combustion of a flammable liquid in a narrow rectangular channel is performed. The emission spectrum of combustion products and the absorption spectrum of butanol-1 are obtained. The case when the temperature of the combustible liquid is lower than the flash point is investigated. A comparative analysis of emission and absorption spectra shows that they do not intersect well, and the radiative heating of the liquid during diffusion flame propagation along the surface cannot be too large.