AbstractThe disappearance rate of diazinon in two different soils, at two levels of water content, with two different diazinon concentrations, and in steam‐treated and untreated soils has been studied. The rates of disappearance were measured by the slopes of the disappearance curves in a semi‐logarithmic coordinate system and by the corresponding half‐lives (RL50 in days) for each combination of the factors.The results were examined statistically, and it was found that all of the four factors influenced the degradation rate of diazinon in soil, the greatest effect occurring with steam‐treatment, i.e. caused by the micro‐organisms in the soils, and the least from the diazinon concentration. Interactions between some of the factors are demonstrated.