This work reports the first pressure stabilization of a hemiketal intermediate, 2-methoxy-2 propanol, formed in the reaction of acetone and methanol, observed using Raman spectroscopy. Acetone dissolved in methanol is found to under go a two-step reaction, with the ketal 2,2-dimethoxy-propane as the final product and the hemiketal observed as an intermediate. In a more dilute stoichiometric mixture of acetone and methanol in liquid tetrahydrofuran (THF), quantitative formation of the hemiketal is observed at pressures above 2 GPa. The complete set of reaction thermodynamic functions (DeltaH degrees, DeltaV degrees, DeltaS degrees, DeltaG degrees, DeltaU degrees, DeltaA degrees) is obtained for the hemiketal formation reaction in THF as a function of temperature and pressure and compared with previous measurements of DeltaH degrees, DeltaS degrees, and DeltaG degrees for acetone dissolved in methanol. Comparisons with quantum calculations for the isolated reactant and product species are used to completely quantitate the effects of solvation on the hemiketal formation reaction.