Metropolis Monte Carlo simulations of hydration effects on rigid solutes characterizing the reactant, transition state (TS), and product for the rate limiting step of an acid catalized rearrangement of an a-acetylenic alcohol to α,β-unsaturated carbonyl compounds are calculated. The model compound corresponds to the protonated 3 methyl-but-1-yne-3-ol. The electronic structure and geometry of the corresponding species are determined with ab initio analytical gradient techniques; a 4-31G basis set has been used. Electrostatic and solute shape effects on samples having 125 MCY-water molecules at 300 K have been examined. Gurney's model for ion-molecule interactions has been adopted. Although the solute-water potential used is very simple, the results on hydration energetics appear to be fairly reasonable. Solute shapes are found to play a significant role in producing differential solvation effects. Electrostrictive effects have been made evident by running MC simulations with fully uncharged solutes. A solvent activation barrier is detected. From the study of the TS solvation structures a possible incidence of ionic strength, counterion presence, and structure-making or breaking solutes can be conjectured. The structural features found for the solvation sheaths of reactant, TS, and product are in excellent agreement with the postulated molecular mechanism. © 1985 American Institute of Physics.