The optical transmission spectra of four ryholitic obsidian samples were measured in order to determine the importance of radiative heat transfer in granite magmas. The spectra, obtained in the temperature range 20-800¿C, show that the radiative spectral window in these samples is limited by a charge transfer band in the UV (400 nm) and Si-O stretching overtone in the IR (4500 nm). Within this window the main obstacles to radiative transfer, in order to decreasing importance, are background scattering, a water band centered at 2800 nm, and an Fe2+ crystal field band at 1100 nm. Unlike crystalline silicates the absorption bands in obsidian do not broaden significantly as temperature increases. As a result, the temperature dependence of the calculated radiative thermal conductivity KR is dominated by the Tβ term. Actual values of KR increase from 9¿10-5 to 1¿1-3 cal cm-1 s-1 deg-1 between 300¿ and 800¿C, the high-temperature value being comparable to the lattice thermal conductivity in obsidian and a lower limit for KR in granitic melts. As the scattering coefficient in melts is probably significantly lower than in obsidian, the radiative conductivity in active plutons is likely to be much higher. As an example, if scattering and the water band are removed from the observed spectra of the obsidian samples, calculated values of KR could increase by a factor of 5, to about 5¿10-3 cal cm-1 s-1 deg-1 at 1000¿C.