The invention relates to a process for combatting pests (rodents, insects) in loose material, especially grains, e.g. cereals. One long-used and effective method of treatment involves gassing with phosphane (PH3) or other pesticides available in gas or vapour form and heavier than air. According to the invention, CO2, preferably in the form of flakes of dry ice, is also caused to penetrate by gravity into the loose material. Although the quantity of pesticide may be greatly reduced, the pesticidal effect acts much more quickly. It is particularly advantageous to use zinc phosphide as the precursor for the pesticide and dry ice as the source or CO2.