MCPA [2-methyl-4-chloro phenoxy acetic acid] is applied with various water sources containing antagonistic salts. It is applied without an adjuvant but is often mixed with bromoxynil for a broader spectrum of weeds. MCPA (K-salt) was applied in carriers containing calcium chloride and sodium bicarbonate. Ammonium salts and other adjuvants were tested to overcome possible antagonism. Control ofFallopia convolvuluswith MCPA was decreased with the calcium chloride as carrier but not when sodium bicarbonate was the carrier. Ammonium sulphate and ammonium nitrate overcame the calcium chloride antagonism and ammonium bicarbonate partially overcame the antagonism. Ammonium hydroxide and potassium chloride could not overcome calcium chloride antagonism, even in the presence of other adjuvants. An alkylated phenol-ethylene oxide surfactant could not overcome antagonism when applied in the absence of ammonium nitrate. Calcium chloride antagonism of MCPA in a mixture with a bromoxynil formulation blank was similar to the antagonism of MCPA applied alone. Both a petroleum oil/nonionic surfactant blend and the alkylated phenol-ethylene oxide surfactant reduced the ability of ammonium bicarbonate to overcome calcium chloride antagonism. MCPA (K-salt) should be registered with ammonium sulphate or ammonium nitrate in areas with hard water.