Measurements of the sorption coefficient for trace amounts of radon in carrier air were made for a variety of porous materials common at the Earth's surface. The technique used was specific for sorption, so some of the ambiguities in previous studies were avoided. Generally, sorption coefficients decrease significantly with increasing moisture and temperature. Model calculations were carried out to relate the temperature dependence of sorption coefficients to the release of radon to the atmosphere. The time-dependence of the exhalation depends on the size of the sample its moisture level and the rate of propagation of heat into the sample. In the steady state limit for thin samples, there is not necessarily an increase in exhalation with decrease in sorption coefficient. Measurements suggest there could be places on the Earth's surface where desorption of radon is directly important for release of radon to the atmosphere. The top several centimeters of dry, unvegetated soil in arid or semiarid climates, and loose rocks upon such soil, might be situations where desorption is important. However, due to the pervasiveness of moisture in soil in moister climate, and at greater depths in soil, desorption's role is probably limited as a major mechanism for release of radon to the atmosphere over much of the Earth's surface. ¿ American Geophysical Union 1989