The purpose of this article is to predict the structure of the CH 3COO − anion in aqueous solution. For this purpose, the following effects of water molecules on the structure of CH 3COO − in aqueous solution are considered: (A) the effect of the first solvent shell; (B) the effect of water molecules around the O′ of CH 3COO −(H 2O′) 6; (C) the effect of water molecules around the CH 3 of CH 3COO −(H 2O) 6; and (D) the effect of water molecules around H′ of CH 3COO −(HOH′) 6. In order to estimate these effects, geometry optimization with 6-31++G* basis sets is carried out for CH 3COO −(H 2O) n ( n=0,1,2,3,4,5,6), CH 3COO −(H 2O) 6(HOH) m ( m=1,2), CH 3COO −(HOH) 6(OH 2) m ( m=1,2), and H 2O…CH 3COO −(H 2O) 6. The vibrational analysis for CH 3COO −(H 2O) n ( n=0,1,2,3,4,5,6) with 6-31++G* was performed at the optimized structures to confirm that all vibrational frequencies are real. The effect of the first solvent shell is significant. The other effects are predicted to be small. Thus, the structure of CH 3COO − in aqueous solution is nearly the same as the structure of CH 3COO − in CH 3COO −(H 2O) 6. The C–C bond length of CH 3COO − in aqueous solution is predicted to be smaller than the C–C bond length of CH 3COO − in the gas phase by 0.023 Å.