Abstract <p>Insecticides like dimethoate and oxydemeton-methyl are essential for crop protection, supporting food production. However, their overuse raises environmental and food safety concerns due to contamination and residues. A new spectrophotometric method was developed to monitor these insecticides in formulations and environmental samples such as water, grains, and vegetables. The proposed method is based on the redox reaction of the thiol (the hydrolytic product formed in an alkaline medium) in both insecticides, with ferric chloride, leading to the formation of Fe²⁺, which subsequently reacts with 1,10<b>-</b>phenanthroline to form colored complexes, measurable at 510 nm. The method obeys Beer’s law within the concentration ranges of 0.46–13.74 µg/mL for dimethoate and 0.49–14.76 µg/mL for oxydemeton-methyl. Reaction parameters such as hydrolysis time, solvent type, heating time, and reagent concentration were optimized to enhance the sensitivity and stability of the method. The recovery results from water and environmental samples demonstrated the good accuracy and precision of the method, with recoveries of 89–100.8% for dimethoate and 89.9–99% for oxydemeton-methyl, and a relative standard deviation of 0.29–1.96 and 0.23–1.91% for dimethoate and oxydemeton-methyl, respectively.</p>