In order to determine the extent and causes of radon variability in soil gas, repeated measurements of 222Rn, 220Rn, and soil properties including moisture, temperature, permeability, and diffusivity have been made at five sites in central Pennsylvania. Concentrations of 222Rn and 220Rn to a depth of 2 m varied temporally in an annual, approximately sinusoidal pattern having an amplitude of twofold to tenfold at all five sites. The existence of the annual pattern is independent of monitoring method, soil drainage, or bedrock parent material. Radon variability is caused by changes in soil moisture content and distribution: the action of soil water on Rn is classified into two distinct regimes: (1) At low and moderate moisture contents, diffusion occurs dominantly in air-filled pores, and Rn is distributed between air and water approximately at equilibrium. Under these conditions Rn in soil air at depth increases with increasing moisture. (2) At high moisture contents diffusion occurs largely in water-filled pores, and air/water equilibrium exists only near interfaces. Under these conditions, Rn in soil air can be low. Since Rn partitioning between gas and water is temperature sensitive and because soil moisture and temperature change in annual cycles, much of the variability in 22Rn concentration occurs in annual cycles. As a result, knowledge of regional and temporal soil moisture and temperature patterns allows estimates of 222Rn concentration in soil gas. In areas where the soil substrate has a significantly lower Ra concentration, emanation coefficient, or dry bulk density than the soil, the substrate can act as a Rn sink with respect to the soil. Under extreme conditions the profile in the soil may actually display a concentration gradient toward the rock. The process of horizon formation during soil genesis significantly affects the vertical distribution of Ra, emanation coefficient and porosity, and therefore Rn production in soil. ¿ American Geophysical Union 1992