Simple carboxylic acids, R-COOH (R = methyl, ethyl, vinyl, phenyl, and hydroxyphenyl), and the monohydrates of acetic and benzoic acids have been studied in the gas phase using ab initio quantum chemical methods. HF/6-31G* and MP2/6-31G* geometry optimizations prove that calculations at the MP2 level reproduce well the available experimental gas-phase structures. In the most stable conformation of the free acids the acid hydrogen is syn to the carbonyl oxygen. Conformers with an anti acid hydrogen are local energy minima for the aliphatic acids and for the nonplanar benzoic acid; however, arrangements with an sp(2) hybridized P carbon and with a planar heavy atom skeleton correspond to transition structures. The anti acid conformers are higher in energy than the syn form by 7-10 kcal/mol obtained in MP2/6-31G*//HF/6-31G* calculations. Results obtained at the MP2/6-311++G**//MP2/6-31G* level for acetic acid and benzoic acids show that the calculated energy separation decreased by 1-2 kcal/mol. The syn/anti rotation barrier for the benzoic acid carboxylic hydrogen was estimated at 12.6 kcal/mol at this level. Monohydration of the acetic acid and benzoic acids leads to small geometry changes for the carboxylic group. The syn monohydrates have a cyclic COOH...OH2 structure with a shorter Ow...H-O(ac) and a longer Ow-Hw...O=C hydrogen bond. This structure is optimal in the gas phase but is less likely in aqueous solution. There is a single Ow..H-O(ac) hydrogen bond for the anti monohydrate. The anti structures are higher in energy than the syn by 7-9 kcal/mol obtained in MP2/6-311++G**// MP2/6-31G* calculations. The energies of association are estimated as -9.5 kcal/mol and -8.2 kcal/mol for the syn and anti monohydrates, respectively.