Resistance to phosphine (PH₃) in key insect pests of stored grain is increasing, with a requirement for maintaining a dose as high as 1 mg l^-1 for 14 d for effective fumigation, which is difficult to achieve under most commercial storage conditions. There is no suitable replacement for PH₃, as most of the available alternatives suffer from specific weaknesses, creating an urgent need to increase the efficacy of this fumigant. One such possibility is co-fumigation of PH₃ with another fumigant, sulfuryl fluoride (SO₂F₂—SF), with the goal of decreasing the time required for a successful fumigation. In this study, adult of two PH₃-resistant strains in each of four key grain insect pests, Rhyzopertha dominica, (F.) (Coleoptera: Bostrichidae) Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), Sitophilus oryzae (L.) (Coleoptera: Curculionidae), and Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae) were fumigated with PH₃ and SF individually as well as in combinations at 25°C over 48 h. Mortality responses in each species were subjected to probit analysis to determine the LC₅₀ and LC_99.9 of PH₃, SF, and PH₃ + SF. Co-fumigation of PH₃ with SF resulted in a 50% reduction of the PH₃ concentration required to achieve 99.9% mortality in two pest species. For example, the PH₃ + SF mixture, reduced the amount of PH₃ required from 14.2–14.5 to 5.6–6.36 mg l^-1 and from 2.71–5.03 to 0.93–1.2 mg l^-1, respectively, for C. ferrugineus and S. oryzae.The overall mortality response to the PH₃ + SF mixture followed an “additive model” suggesting that mutual enhancement in toxicity can be achieved with this mixture specifically to control PH₃-resistant insects.