Surface temperatures of a 1989 andesite lava flow at Volc¿n Lonquimay, Chile, derived from an analysis of Landsat Thematic Mapper (TM) infrared data, show a progressive cooling of crusted lava from 250ℏC to 170¿C down a 1.5 km segment of the flow. These results are obtained by solving ''dual-band'' equations for the two short-wavelength infrared TM bands, assuming that the lava surface can be respesented by two thermal components, exposed core and chilled crust, and unlike previous investigations, by predetermining the temperature of the hot component rather than the cool component. This approach is consistent with both observational data and theoretical models, and shows that any exposed core of the flow occupies tiny fractions, possibly less than 0.03%, of the surface area. As a result, the crust made by far the greatest contribution to radiative heat losses, estimated at between 1 and 3 MW per 30¿30 m pixel, along most of the flow's length. Convective heat transfer into the air above the flow is about two thirds the corresponding radiative flux. These estimated surface heat losses suggest thicknesses of less than 1 m for a conductive thermal boundary layer component of the crust. Radiance values in the thermal infrared band 6 of the TM image are consistent with temperature derived from the short wavelength infrared data and reveal the thermal manifestation of a topographic step beneath the flow. ¿American Geophysical Union 1991