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The title compound, C12H11N2+.C2H3O2-.H2O, is a hydrated salt of 2-methylperimidinium acetate. The 2-methylperimidinium cations are protonated at the N atoms. In the acetate anions, the C-O bond distances are 1.238 (3) and 1.265 (3) Å. A hydrogen-bonding network allows the self assembly of the ionic counterparts together with the water of hydration along the c axis.