Glyphosate-based, ready-to-use weed control products frequently contain diquat (typically, 0.04 by weight relative to glyphosate) under the supposition that the diquat, “makes glyphosate work faster.” However, in light of the known modes of actions of glyphosate and diquat, we hypothesize that diquat may be antagonistic to glyphosate activity. Greenhouse experiments using longstalked phyllanthus were conducted to test this hypothesis. Glyphosate was applied at a series of rates, ranging from 0.11 to 1.12 kg ae/ha, either alone or tank-mixed with either 0, 0.02, 0.04, and 0.06 diquat. Onset of visual injury was more pronounced with the glyphosate + diquat tank mixtures compared with glyphosate alone. However, long-term control, as expressed by regrowth suppression, was greater with glyphosate alone. Regression analysis indicated that, at marginally effective glyphosate rates, the amount of glyphosate must be increased by approximately 60% to compensate for the diquat-based antagonism. Absorption and translocation studies using 14C-glyphosate revealed that the antagonism of diquat toward glyphosate can be attributed to reduced translocation of absorbed glyphosate.