The heat of formation of liquid methanesulfonic acid, -178.09 ± 1.48, was determined by measuring the heat of reaction of methyl thiolacetate with aqueous hypochlorite solution to give aqueous methanesulfonate and acetate. The heats of formation of liquid methanesulfonyl chloride, -126.91 ± 1.54, and methyl methanesulfonate, -164.34 ± 1.58, were determined by measuring the heats of reaction of methanesulfonyl chloride with water or methanol in the presence of a suitable basic catalyst. Heats of vaporization (based on vapor-pressure data), entropies (based on ab initio molecular orbital calculations), and free energies of transfer from gas phase to aqueous solution were calculated leading to values for the free energies of formation in aqueous solution. The free energies of formation so determined were methanesulfonic acid, -151.72 ± 2.68, methanesulfonyl chloride, -101.29 ± 1.96, and methyl methanesulfonate, -127.28 ± 2.08. From these values the free energies of hydrolysis (leading to unionized methanesulfonic acid) are methanesulfonyl chloride, -25.11 ± 3.04, and methyl methanesulfonate, -9.90 ± 2.48.Key words: sulfonic acids, heat of formation, free energy of formation, hydrolysis.