The capacity of three adjuvants (Freeway, LI-700 and ammonium sulphate) to overcome the negative effect of a post-application rainfall on glyphosate efficacy was studied. Ten to twelve leaves purple nutsedge plants (Cyperus rotundus L.) were sprayed with glyphosate (1,92 and 2,88 kg ia·ha-1) alone and with the adjuvants. Eight and sixteen hours after herbicide treatments were sprayed, a simulated rain (12,5 mm in 20 min) was applied to purple nutsedge plants. For each herbicide treatment, plants that did not receive rainfall after their application were left for comparison. Also, check plants, which were not sprayed with glyphosate were included. Results showed; a) rainfall after glyphosate application significantly decreased herbicide efficacy, b) adjuvants did not increased purple nutsedge control with glyphosate, under no rain conditions and, c) the silicone surfactant Freeway was able to overcome, almost totally, glyphosate efficacy, when submitted to rainfall after its application.