Aqueous acid dissociation constants of substituted areneseleninic, areneselenenic arenesulfinic and benzoic, acids are calculated by ab initio (MP2) and DFT (B3LYP) methods in combination with bulk solvation models (IEFPCM CRSrad) from appropriate thermodynamic cycles Mean absolute deviations (MAD) between experimental and calculated pK(a) values are quite large for basis sets without diffuse functions however trends are reasonably well described Best agreement with experiment as described by MAD as well as correlation coefficient and slope of the correlation equation pK(a) = a*Delta(calc)/RT In(10) + b is obtained with the CPCM solvation model using the defaults optimized within COSMO-RS (CRSrad MAD = 1 54 R-2 = 094, a = 0 83) Sulfenic (sulfenic) acid tautomers are significantly more stable than the corresponding sulfoxide (selenoxide) forms