Many of the commercial fumigants, methyl bromide, chloropicrin, dichlorovos, have environmental problems, and some of them will be phased out. New alternative fumigants need to be developed for safety, biodegradability and selectivity. This research investigated fumigation toxicities of natural and synthetic cyanohydrins against stored-product pests and house fly. Using fumigation toxicity of cyanohydrins to the house fly and the lesser grain borer, quantitative structure-activity relationships (QSAR) were determined. For evaluating what components kill the insects, mode of action of cyanohydrins in insects were also conducted.;Most natural and synthetic cyanohydrins were as effective as or more effective than commercial fumigants against stored-product pests and the house fly. Log P, polarizability and molar refractivity, which are classical parameters for explaining toxicity, were well correlated with the fumigation toxicity of cyanohydrins against the house fly and to a lesser degree for the lesser grain borer. Small quantities of cyanohydrins were detected in the headspace of the experimental chamber and in insects. The total cyanide ion in cyanohydrin-exposed insects was less than in hydrogen cyanide-exposed insects, but some cyanide ion was released in vivo to be toxic to the cyanohydrins-exposed insects.;As a result of this research, it seems feasible that natural and synthetic chemicals might be used as alternative fumigants because of their insecticidal activity against stored-product pests and the house fly. Risk assessments, however, should be done to evaluate mammalian toxicity, environmental safety and human health. The QSAR study explained the interactions between the structures of natural and synthetic cyanohydrins and their biological activities. Finally, this research supports the assumption that naturally occurring cyanohydrins can be degraded to release free cyanide ion in vivo, confirming that those cyanogenic compounds kill insects.