The hydration of carbonyl compounds, along with hemiketal and ketal or hemiacetal and acetal formation, has been studied both experimentally and theoretically. The heats of hydrolysis of some acetals and ketals were determined calorimetrically. Equilibrium constants for hydrate, hemiacetal, and acetal formation were determined via NMR spectroscopy, and heats of reaction were obtained from the change in equilibrium constants with temperature. Calculations of the hydration energies were made using a set of theoretical models through MP2/6-311++G**//HF/6-31G* for formaldehyde, acetaldehyde, acetone, cyclopropanone, and cyclobutanone and through MP2/6-31G**//HF/6-31G* for crotonaldehyde, bicyclo[1.1.1]pentan-2-one, cyclohexanone, 7-norbornanone, methyl acetate, chloral, and fluoral. Additional calculations were carried out using the CBS-4 theoretical model. The energies of hemiacetalization and acetalization also were calculated for formaldehyde, acetaldehyde, and acetone. The origin of the changes in hydration energies was analyzed using group transfer reactions of formaldehyde with an alkane to give the substituted carbonyl compound and of formaldehyde hydrate with an alkane to give the substituted carbonyl hydrate.