The present study was aimed to investigate how salinity and pH of water which carries insecticides on their efficacy. Physico-chemical properties [salinity%, TDS mg/L, ECms and pH] of four water quality, ie., Nile water (N.W), Agricultural drainage water (Adw), Ground water (G.W, 16m³ deep) and distilled water (d.w) were measured alone and with marshal 20% EC, Sumithion 50% EC and oshin 20% SG. The LC₅₀ values and both initial and residual activities of these insecticides with different kinds of water were also determined under laboratory and field conditions against the cotton aphid, Aphis gossypii. The results clearly indicated that both N.W and AdW had high level of salinity and pH, therefore were considered as "Hard" according to standard classification system of Analytical services Litchlab. In addition, all the tested insecticides confirmed lowest toxicity (LC_50­) and lowest initial and residual effects when these insecticides diluted in both Nile water and agricultural drainage water. These effects could be attributed to such water contains high level of cations (positively charged) such as Ca⁺², Mg⁺², Fe⁺³, Na⁺ and other ions attach to negatively charged insecticide molecules then, render the compound inactive. In addition insecticides particularly organophosphates and carbamates, undergo chemical breakdown in alkalin water (pH>7). So that salinity and pH in spray water can diminish the effectiveness of insecticides. On the other hand, turbidity water has soil particles (clay and silt), these particles can bind with insecticide molecules and harbor microbes that naturally degrade insecticide, and clog Filters and nozzles.             Generally how to keep it from becoming a problem? Please check the water pH before adding any compound. Water should be clean and clear for all pesticide applications. Remember read the pesticide label for recommended pH range.