Esters of substituted phenoxy‐phenoxy propionic acid constitute a new class of herbicides that are effective against gramineous weed and crop species. Slight changes in chemical structure alter drastically the spectrum of weeds controlled by this class of herbicides. Wheat (Triticum aestivumL.) is resistant to diclofop‐methyl (methyl 2‐[4‐(2′,4′‐dichlorophenoxy)phenoxy] propanoate) (DM) and clofop‐isobutyl (iso‐butyl 2‐[4‐(4′‐phenoxy)phenoxy]propanoate) (CI), oat (Avena sativaL.) and wild oat (Avena fatuaL.) are susceptible to DM but resistant to CI, and corn (Zea maysL.) is susceptible to both compounds. The antagonism of IAA‐induced elongation in the coleoptile straight growth test was determined to measure biological activity of the herbicides. The basis for the differential responses by gramineous species was related to the metabolism and deioxication of the herbicides in coleoptiles. Growth of wheat coleaptiles was relatively unaffected by both compounds, oat coleoptile growth was inhibited by DM but not by CI. but corn coleoptile growth was inhibited equally by both compounds. Coleoptiles and excised shoots of the three species rapidly hydrolyzed both compounds to their respective acids (diclofop, clofop). The acids were conjugated to a water‐soluble ester conjugate or they were hydroxylated in the chlorine‐substituted phenyl ring and conjugated as a phenolic conjugate. Aryl hydroxylation is a detoxication mechanism in resistant plants. Plants resistant to DM or CI contained low concentrations of the parent ester and the free or bound (ester conjugate) acid and a high concentration of free or bound (phenolic conjugate) aryl hydroxylated acid in coleoptile and shoot tissues, Differential responses by the three gramineous species to DM and CI axe due apparently to differences in their detoxication mechanism. The enzyme for aryl hydroxylation in oat appears to have a higher affinity for the 4‐chloro‐ than for the 2,4‐dichloro‐substituted moiety. Therefore, oat hydroxylated clofop rapidly and was tolerant to CI but the limited ability of oat to hydroxylate diclofop resulted i