Photosystem II (PS II) inhibitors halt electron flow within the photosynthetic electron transport chain, thereby leading to increased oxidative stress. As a result, their addition to mesotrione, which inhibits carotenoid biosynthesis by inhibition of the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD), is complementary. Field and greenhouse experiments were conducted in 2002 and 2003 to investigate the joint action of POST mesotrione plus PS II inhibitor herbicide combinations. The joint action of mesotrione plus PS II inhibitors was investigated across five plant species, three PS II inhibitors, and two moisture environments to determine their influence on the joint action response. Rates of mesotrione evaluated ranged from 4.4 to 87.6 g ai/ha alone and in combination with reduced rates of atrazine, bromoxynil, and metribuzin. In the field, all combinations of mesotrione at 8.8, 17.5, and 35.0 g/ha plus atrazine, bromoxynil, or metribuzin were synergistic for necrosis 6 d after treatment (DAT) on sunflower. Addition of atrazine at 280 g/ha to mesotrione at 8.8 g/ha increased velvetleaf leaf necrosis by 18 to 47%. In the greenhouse, the addition of bromoxynil at 70 g/ha to mesotrione at 17.5 g/ha increased leaf necrosis by 23 to 34% and biomass reduction by 38 to 47%. Synergism on Palmer amaranth occurred similarly under both normal and dry moisture environments at application. Plant height at application was found to influence detection of synergism on the whole-plant level. Nomenclature: Atrazine; bromoxynil; mesotrione; metribuzin; Palmer amaranth, Amaranthus palmeri (S.) Wats. #3 AMAPA; velvetleaf, Abutilon theophrasti Medicus # ABUTH; tame sunflower, Helianthus annuus L. # HELAN. Additional index words: Interaction, joint action, synergism, carotenoid biosynthesis inhibitor, photosynthesis inhibitor, triketone herbicides. Abbreviations: ADM, additive dose model; DAP, days after planting; MSM, multiplicative survival model; PET, photosynthetic electron transport; PPFD, photosynthetic photon flux density; PQ, plastoquinone; RCBD, randomized complete-block design; SWC, soil water content.