Petroleum solvents and oils commonly are used as formulants and adjuvants with pesticides. Information on plant response to solvents and oils could assist in selection of the most efficacious formulation and/or adjuvant for a herbicide. A technique was developed to study the interaction of solvents and oils with leaf wax from a variety of common weeds. Some of the 20 solvents and oils studied recoiled or contracted from leaf wax; recoil did not correlate with solvent phytotoxicity. Wax solubility in a given solvent differed depending on plant species. All solvents that caused leaf injury readily dissolved leaf wax, but not all solvents that readily dissolved leaf wax caused plant injury. The relative solubility of leaf wax in various solvents is influenced by solvent volatility and contact time. Emulsifier generally did not affect relative wax solubility in the solvents, but did increase the phytotoxicity of solvents applied as spray carriers. Solvents with emulsifier applied as spray adjuvants were less phytotoxic than solvents applied with or without emulsifier as spray carriers. In efficacy studies, herbicides evaluated were bentazon, fluazifop-P, quizalofop and sethoxydim, all developed for weed control in soybeans. Solvents or oils applied as herbicide spray adjuvants or carriers differed in their enhancement of herbicidal efficacy depending upon herbicide and plant species. The lack of a consistent relationship between relative wax solubility and solvent effectiveness as a herbicide spray adjuvant or spray carrier indicates that other factors may be involved in enhancing herbicide efficacy.