A theoretical study of propylene oxide acid-catalyzed hydrolysis was performed by investigation of the S(N)1 and S(N)2-like mechanisms. By using chemometric tools, hierarchical cluster analysis (HCA), and principal component analysis (PCA), the MP2/6-311++G** level of theory was selected from HF, MP2, and DFT as the best method to describe the geometry of the basic skeleton (oxirane). At this level of theory, geometry optimizations, vibrational frequencies, intrinsic reaction coordinate (IRC), and other thermodynamic calculations have shown that the borderline S(N)2 mechanism is more favorable than pure S(N)2 and S(N)1 mechanisms in the gas phase. In the S(N)1 mechanism, the existence of the typical carbocation was not observed, and furthermore, the possibility of epoxide conversion to a protonated aldehyde was indicated, even in the presence of a water molecule (nucleophile). The Chelpg charge distribution of the reactants, steric hindrance, synchronous bond breaking-formation and trajectory angle of nucleophilic attack are discussed for pure and borderline S(N)2 mechanisms. Solvation effect calculations indicate that the pure S(N)2 mechanism is more favorable than borderline S(N)2 and S(N)1 mechanisms. This is discussed in terms of hydrogen bond formation.