The joint action of binary mixtures of the amino acid biosynthesis-inhibiting herbicides glyphosate, glufosinate-ammonium, metsulfuron-methyl and imazapyr was assessed in pot experiments applying the Additive Dose Model (ADM). Plants of Sinapis arvensis or S. alba were sprayed with seven doses of the herbicides alone and binary fixed-ratio mixtures of the four herbicides. In total, 73 binary mixtures were studied in six separate experiments. Mixtures of glyphosate and glufosinate-ammonium were less phytotoxic than predicted by ADM whether commercial formulations or technical grade products were applied. In contrast, mixtures of glyphosate and metsulfuron-methyl, glyphosate and imazapyr, glufosinate-ammonium and mets-ulfuron-methyl, glufosinate-ammonium and imazapyr, and metsulfuron-methyl and imazapyr either followed ADM or were synergistic. Synergism was observed most frequently for mixtures of glyphosate or glufosinateammonium with metsulfuron-methyl. Synergism was also more pronounced for commercial formulations of glyphosate and glufosinate-ammonium than for the corresponding technical grade formulations, implying that synergism was caused by the presence of the formulation constituents of the commercial glyphosate and glufosinate-ammonium formulations in the spray solution.