SUMMARYNon‐steady‐state diffusion of chemicals in soil can be described by Fick's second law using suitable effective diffusion coefficients. In this study a ‘stopped elution’ method was used to determine experimentally these effective diffusion coefficients for gaseous and dissolved substances in dry and water‐saturated soil. For this purpose a high‐performance liquid chromatography (HPLC) apparatus and a gas chromatograph (GS) were used in combination with soil‐filled columns. The method is based on the observed spreading of a substance moving through a soil column. This broadening is the result of several processes, one of which is diffusion. A suitable experimental design permits the separate measurement of that part of the widening caused by diffusion.The measured effective diffusion coefficients were compared with theoretical values calculated using predictive algorithms from the literature. For liquid‐phase diffusion the maximum deviation between the calculated and the measured values was 34%. The experimental results for gaseous diffusion showed systematic upward deviation from the calculated Coefficients. This might be caused by the diffusion of molecules in the adsorbed state, a phenomenon that had been neglected in t