Abstract Multispectral satellite data recorded in the short-wavelength infrared (SWIR) region have been used to estimate temperatures of hot volcanic features such as fumaroles and lava bodies. This has required the assumption that thermal distributions across the surfaces in question can be adequately represented by a very few (usually two) temperature components. We investigate here the applicability of such simple descriptions to fumarole fields, an important class of thermal phenomena. Our surface temperature surveys, conducted at Momotombo (Nicaragua) and Vulcano (Italy), suggest that SWIR emission from the interiors of high-temperature fumarole vents far exceeds that from their surroundings. In contrast, cooling crust on active lava bodies can contribute more SWIR radiation than the small areas of exposed incandescent material that are often present. Such a distinction should be discernible in the relative response of the two Landsat Thematic Mapper (TM) SWIR bands, providing a basis for identification of the volcanic nature of thermal anomalies in Landsat TM or similar data, even in the absence of field observations. However, estimates of subpixel temperatures and heat flux, made from such satellite data, must be considered unreliable.