Phenyl-substituted analogues of 2-(phenylmethyl)sulfonylpyridine 1-oxide preemergent herbicides were examined in order to determine quantitative relationships between structure and activity against the following three weed species: switch grass (Panicum virgatum L.), barnyard grass (Echinochloa crusgalli L. Beauv.), and green foxtail (Setaria viridis L. Beauv.). Analogues were chosen to provide maximum parameter orthogonality. Regression analysis yielded structure-activity relationships wherein the most significant substituent parameters associated with herbicidal activity were found to be the partition coefficient (pi), the molar refractivity (MR), and two indicator variables, Z (denoting the presence of an alpha-methyl group) and H (denoting an ortho substituent capable of hydrogen bonding). For green foxtail, the structure-activity relationship was found to be: -log ED50 = 0.43 pi -0.052MR + 0.50H + 0.24Z + 0.61, where ED50 is expressed in moles per acre. The regression equations were found to explain 79-93% of the bioactivity for the three weed species studied. It was further shown that these equations represent the best possible correlations within the limitations of the biological data.