The sorption and desorption of PH₃ from wheat treated with concentrations of 0.5-5 mg/kg of wheat was determined at 25, 45 and 85°C. The desorbed PH₃ was extracted from the air by passing through a cold trap; here sufficient quantities of the gas could be accumulated for accurate analysis by GLC. Most of the PH₃ was desorbed in the first 2-3 days but small amounts continued to desorb for many weeks following treatment. After 220 days of aeration, PH₃ was still present in the grain and desorbing at the rate of ppt (10* h-12*s g). For a wide range of concentrations, about 10per thousand of the PH* l3*s applied was found to be adsorbed. The amount of physically sorbed PH₃ increased with the fumigant concentration applied and with the length of exposure. Because of reduced reactivity even when the temperature was increased to 85°C for several days, unreacted PH₃ still desorbed slowly from wheat. Corn aerated 26 days desorbed 0.004 ng/g in 2 days.