Theoretical study of the acetic acid dimer, its microhydration and its behavior in water and chloroform solution was performed. To characterize the system, we adopted ab initio methods at the DFT and RI MP2 (the resolution of the identity approximation MP2) levels for the gas-phase calculations, PCM (polarizable continuum model) approximation using the polarizable conductor calculation model (COSMO) for description of solvent, and constant energy (NVE) and constant temperature (NVT) molecular dynamics simulations for gas phase and explicit solvent calculations, respectively. The cyclic structure of the acetic acid dimer is the most stable in the gas phase only. During microhydration, the water molecules are incorporated in the dimer leading to water-separated structures. This conclusion is based on ab initio quantum chemical calculations, as well as on molecular dynamics simulations. The fact that the cyclic structure does not appear in water solution is in agreement with previous theoretical and experimental results. Extending the search also on other acetic acid dimer structures, we concluded that acetic acid does not form any dimer structure in water solution. The cyclic structure is also supposed to be stable in chloroform solution.