The basis for selectivity of phenmedipham (methyl-m-hydroxycarbanilate m-methylcarbanilate) and desmedipham (ethyl m-hydroxycarbanilate carbanilate) on wild mustard [Brassica kaber (DC.) L.C. Wheeler ‘pinnatifida’ (Stokes) L.C. Wheeler], redroot pigweed (Amaranthus retroflexus L.), and sugar beet (Beta vulgaris L.) was studied by evaluating spray retention, absorption, translocation, and metabolism. Total photosynthesis in wild mustard was severely inhibited in less than 5 hr after foliar application of either herbicide and did not recover. Total photosynthesis in sugar beet was slightly inhibited but recovered after 24 hr. Photosynthesis in redroot pigweed recovered from a treatment of phenmedipham but did not recover when treated with desmedipham. Differences in spray retention or foliar absorption did not explain selectivity. Within 5 hr after herbicide application, redroot pigweed had translocated more desmedipham than phenmedipham from the site of absorption and had metabolized a large amount of the phenmedipham but little desmedipham. The key factor explaining selectivity appeared to be at the initial detoxication reaction of the parent compound.