Abstract: Residues and decay of four insecticides; pirimiphos-methyl, chlorpyrifos-ethyl, cypermethrin and fenvalerate, were determined in three types of water; distilled water (pH 6.4), well water (pH 7.6) and standard hard water (SHW, pH 7.8). These insecticides were added to the three types of water at recommended rates that used to control the insect pests on crops. Samples of water were taken from treated types of water after zero, 7, 14, 21 and 75 days. Water samples were extracted and cleaned-up then analyzed by using gas chromatography for determining the insecticide residues. The data showed that the decay of pirimiphos-methyl after one week was (49.9%) in well water compared with that of distilled and SHW (29.4 ad 44.5%). Decay of chlorpyrifos-ethyl in distilled water after one week was 11.7% then increased afterwards and reached 29.5, 40.8 and 99.5% of the amount determined at zero (initial deposit) after 14, 21 and 75 days. Same trend of chlorpyrifos-ethyl decay was observed in well and SHW. Pirimiphos-methyl was vastly degraded when compared with chlorpyrifos-ethyl in the three different types of water. The data also showed that the decay of cypermethrin was high after one week in the three types of water where it exceeded 75%. On the other hand, the decay of fenvalerate after one week was high in well and SHW water (67.8 and 67.7%) compared with that of distilled water (48.8%). Further, decay of synthetic pyrethroid insecticides (cypermethrin and fenvalerate) was relatively high after one week when compared with organophosphorus insecticides (pirimiphos-methyl and chlorpyrifos-ethyl) in the three types of water.