Studies were made to determine the influence, during fumigations, of moisture and soils upon the conc. of CH3Br in the atmosphere of the chamber and upon the wt. of CH3Br sorbed by the soils. The rate of decrease in CH3Br conc. was found to be almost the same in dry and saturated atmospheres. Three soils[long dash]sand, clay, and peat[long dash]were exposed in a chamber charged with CH3Br at the rate of approx. 2 lb. per 1000 cu. ft. After 6 hrs. the conc. in the chamber loaded with dry soils was 102, 83, and 59%, respectively, of that in the empty chamber. With wet-soil loads the concs. were higher in each case. Dry peat sorbed 41% of the charge or about 4 times as much as sand, and dry clay sorbed 25%. Dry soils sorbed more than wet soils. Consideration should be given to the influence of these factors on the effect of CH3Br on plants and its insecticidal value during fumigation and post-fumigation periods.