Data from remote sensing satellites operating in the short-wavelength infrared can be used to measure temperatures of about 150°C and above. The gain settings of the Landsat Thematic Mapper (TM) sensors are such that to record pixel-integrated temperatures from 150°C to over 1000°C requires the use of several spectral bands. If the radiant source occupies less than a whole pixel, as is the case for fumaroles or a crusted lava surface with incandescent cracks, then the pixel-integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength. By determining pixel-integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel. In some cases our observations provide the only available data on the state of activity of a particular volcano; in others they support and refine field observations. Our Landsat TM data reveal otherwise unobserved precursor and subsequent activity related to the September 1986 eruption of Lascar volcano (Chile) and supplement other data on the March 1986 eruption of Augustine volcano (Alaska). They also indicate the continued presence of lava lakes at Erta 'Ale (Ethiopia) in the absence of any ground-based observations and document minor eruptive activity at Mount Erebus (Antarctica). Landsat Multispectral Scanner data have been used to measure the variation in temperature along a lava flow on Sierra Negra (Galapagos Islands) which occurred in 1979.