Atrazine and metolachlor co‐application to soil created competitive sorption between the two herbicides, producing smaller partitioning coefficients than for separate atrazine and metolachlor applications. The partitioning coefficients for both compounds were also less when applied in a commercial formulation, compared with separate analytical‐grade applications. Since the Freundlich slope of the isotherms was less than unity (0.83 to 0.87), the sorption strength of the herbicides decreased with increasing solution concentrations. Based on the Freundlich adsorption values obtained, this study concludes that atrazine and metolachlor co‐applications on agricultural fields have the potential to increase the mobility of these herbicides in soil, thereby also increasing the risk for groundwater contamination.