Intensive application of pesticides in the agricultural sector and for domestic purposes has resulted in increased usage over the years. Pesticides are used to control pest, diseases and weeds in agricultural and urban areas, but their persistence in the environment has resulted in human poisoning, health risk problem and environmental pollution due to their ability to permeate the soil surface, groundwater systems and water surface bodies. Sorption of two organophosphorus pesticides: Diazinon and Clorpyriphos was studied in a soil (S) and in soil modified with clay addition. Soil Alhue, VI region, Chile, was spiked with 1% of either Montmorillonite (M) or Kaolinite (K). In addition, organic matter (OM), in the soil was eliminated and this soil was spiked again, with 1% of both clays. Batch sorption and kinetics experiments were conducted to obtain the retained amounts onto the soil samples. Pesticides were quantified by high performance liquid chromatography (HPLC). The results showed that for both pesticides the sorption order is: S-M ˃ S-K > S. For soil without OM the order is the same, although the adsorption was lower. The adsorption isotherms were expressed by the Langmuir and Freundlich models. The negative Gibb’s free energy change (ΔG°) values obtained suggest that the adsorption of both pesticides, is an exothermic process. Clay addition to the soil, increased the adsorption processes, generating an increase in the amount of pesticide retained in the soil and partially avoiding possible contamination of the aquifers. The difference found in relation to the behavior of both clays on the soil, can be explained considering its structure and its octanol-water partition coefficient (kow).