SummaryAssessment of gas diffusivityin situgives a direct measure of the ability of soils to exchange gas with minimal soil disturbance. A versatile, readily portable probe for measuring the diffusion of a tracer gas through soilin situis described. The radioactive tracer85Kr is injected into a cell located at the end of the probe. The change in activity within the cell as the gas diffuses out is measured by a Geiger‐Muller tube in the cell. The probe can be used by insertion either directly into an auger hole (buried‐probe mode) or into a chamber pushed into the soil surface. A method to simulate diffusion numerically using Fick's equation for both methods of insertion is presented. In the tests reported, diffusivity was estimated by expanding or contracting the time axis of the simulation until it matched the observed count rates. A goodness‐of‐fit was attached to each diffusivity estimate. The probe was generally effective, giving diffusivities comparable to those measured in the laboratory on cores taken near the cell (buried‐probe mode) or linked to the surface chamber. Poor fits were found for some diffusivities measured in the buried‐probe mode on coarsely structured soils. These were attributed to non‐uniform distribution of porosity and possible upward leakage of tracer when used at shallow depths in the buried‐probe mode. However, thein situdiffusivities may be more representative than those measured in cores in the laboratory because of the greater sample volume. We show how the probe can be used to detect soil layers that restrict gas diffusivity and differences in aeration status between til