Adults of Tribolium confusum Jacquelin duVal and T. castaneum (Herbst) (Coleoptera: Tenebrionidae) and Oryzaephilus surinamensis (L.) (Coleoptera: Cucujidae) were exposed at four relative humidities to binary mixtures of oxygen and nitrogen or to ternary mixtures of oxygen, nitrogen, and carbon dioxide. In most instances, mortality increased with decreased relative humidity when the insects were exposed for limited periods of time to either the binary or the ternary mixtures.