The intrinsic gas-phase acidities of a series of 21 Bronsted acids have been predicted with G3(MP2) theory. The G3(MP2) results agree with high level CCSD(T)/CBS acidities for H2SO4, FSO3H, CH3SO3H, and CF3SO3H to within 1 kcal/mol. The G3( MP2) results are in excellent agreement with experimental gas-phase acidities in the range 342-302 kcal/mol to within < 1 kcal/mol for 14 out of 15 acids. Five of the six acids in the range of 302-289 kcal/mol had an average deviation of 5.5 kcal/mol and the strongest acid, (CF3SO2)(3)CH, deviated by 15.0 kcal/mol. These high-level calculations strongly suggest that the experimental acidities in this very acidic part of the scale need to be remeasured. The CCSD(T)/CBS (mixed exponential Gaussian) additive approach for CH3CO2H, HNO3, H2SO4, CH3SO3H, FSO3H, and CF3SO3H gives excellent agreement (+/- 1kcal/mol) with experiment for the Delta H-f(0)' s of non-sulfur containing species, and supports the low end of the experimental values for H2SO4 and FSO3H. Use of a larger basis set (aug-cc-pV5Z) in the CBS extrapolation improves the agreement with experiment for both H2SO4 and FSO3H. The G(3)(MP2) heats of formation for RSO3H molecules tend to be underestimated as compared to the CCSD(T)/CBS approach by 2.5-7.0 kcal/mol. COSMO solvation calculations were used to predict solution free energies and pK(a) values with pK(a)'s up to -17.4. Including the solvation of the proton gives good agreement with experimental pKa values in the very acidic regime, whereas it is less reliable for weaker acids. The use of CH3CO2H and HNO3 as reference acids in the less acidic and more acidic regions of the scale, respectively, provided improved results to within +/- 2 pK(a) units in nearly all cases (+/- 3 kcal/mol accuracy).