Experiments were conducted to obtain a better understanding of the role of pH on the availability of fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl) phenyl]-4(1 H)-pyridinone) in soil solution when used as a selective herbicide and the partitioning into aqueous and sediment phases when employed for aquatic plant control. Phytotoxicity of fluridone to seedling sorghum (Sorghum bicolor L.) plants increased with increasing pH of the sand-nutrient solution medium. Since stability and plant uptake of fluridone by bioassay plants were not affected by solution pH, the increasing phytotoxicity at basic pH was attributed to less adsorption and hence higher availability of the herbicide in solution. Soil adsorption studies with 14C-fluridone confirmed this trend, as the soil solution concentration at equilibrium increased from 0.091 to 0.258 μg mL−1 and from 0.216 to 0.354 μg mL−1, respectively, as pH of a sandy loam and silty clay loam increased from 3 to 9. In contrast, adsorption on the sandy loam and silty clay loam for the same pH range decreased from 4.108 to 2.435 μg g−1 and from 2.850 to 1.484 μg g−1, respectively. Smaller but significant changes in adsorption were also observed for an organic soil over this range. Key words: Herbicide, fluridone, pH, uptake, soil adsorption