Knowledge of the relationship between DP/D0 (diffusion coefficient in soil divided by diffusion coefficient in free air) and the volumetric soil-air content, ɛ, is important when modeling gaseous movement of volatile organic compounds (VOCs) in soils. The effective diffusion (i.e., diffusion and retardation) of trichloroethylene (TCE), toluene and freon in Yolo silt loam (fine-silty, mixed, nonacid, thermic Typic Xerorthent) were measured in a two-chamber diffusion apparatus. The experiments were conducted on packed soil cores over a range of water contents. Vapor retardation factors were calculated from soil parameters and equilibrium partition coefficients. Partition coefficients were measured in batch experiments. It was found that for water contents higher than four molecular layers of water surface coverage, solid/vapor partition coefficients, KD′, were consistent with values predicted by Henry's Law constants (KH), and aqueous/solid partition coefficients, KD. For less than four molecular layers of water, sorption increased by orders of magnitude. The vapor retardation factors, along with the measured effective diffusion, allowed a calculation of diffusion coefficients (DP) for the investigated species by using the analytical solution to diffusion in a two-chamber apparatus. Values of the ratio DP/D0 were generally higher than the values predicted by the Millington-Quirk equation, and lower than the values predicted by the Penman equation. Compared with the nonreactive tracer freon, DP/D0 values for TCE and toluene agreed very well for higher water contents. Values obtained for air-dry soil, however, were under-predicted. The experimental work for determination of the effective diffusion of reactive tracers can, therefore, for sufficiently high water contents be limited to the determination of Dp/D0-ɛ relations for a nonreactive tracer and measurement of KD, KD′, and KH values for the reactive tracers.