Forty-two mol percent sulfuric acid is a room-temperature molten salt composed of H-bonded ion pairs H3O+- HSO4- (8.2 M) and H5O2+-HSO4- (5.4 M), the former breaking primarily below similar to360 degreesC and the latter, mainly between similar to360 degreesC and the critical point. These ruptures produce H2SO4 and H2O. Iterative van't Hoff thermodynamic analysis of Raman spectra to 360 degreesC employing peak height or component area ratios at similar to905 and similar to1035 cm-(1) or at similar to1165 and similar to1035 cm(-1) yielded an average DeltaH of 3.2 +/- 0.2 kcal/mol corresponding to H-bond breakage between H3O+ and HSO4-. A Raman DeltaH of 5.9 +/- 0.3 kcal/mol was measured for the rupture of H5O2+-HSO4- H-bonds between 360 and 513 degreesC versus similar to5.7 kcal/mol calculated. H-bonds exist between species other than the two hydronium-bisulfate ion pairs but are not measured by the Raman H2SO4/HSO4- intensity ratios. Hydrogen bonding wanes near the critical point, similar to665 degreesC, at which only neutral species, H2SO4 and H2O (plus some SO3 remain. A DeltaH of 3.5 +/- 0.4 kcal/mol was also measured from the 87 mol % acid, close enough to the 3.2 +/- 0.2 kcal/mol value to assign both to H3O+-HSO4- and also because H5O2+ cannot form in the 87 mol % solution. The integral heat of solution (42 mol %) calculated using Raman data for the two hydronium-bisulfate ion pairs is consistent with the calorimetric value. Values of the dielectric constant were also estimated from Raman data.