A number of calculations of acidic As(III) and As(V) species formed with sulfur and oxygen (H3AsS3, H2AsS3-, HAsS32-, AsS33-, H3AsS4, H2AsS4-, HAsS42-, AsS43-, H3AsO3, H2AsO3-, HAsO32-, AsO33-, H3AsO4, H2AsO4-, HAsO42-, and AsO43-) are presented. pK(a)s for Successive deprotonations in both the gas phase and aqueous solution (using both explicit water molecules and a self-consistent reactive field conductive polarizable continuum model (SCRF CPCM) for solvation) are fitted to known experimental values for the H3PO4 and H3AsO4 series' of deprotonations with a linear extrapolation showing, r(2) values of 0.97 for the CBS-QB3 method with a single explicit water molecule in the CPCM. Though the unfitted pK(a)s of H3AsO4 (pK(a1) = 4.6. pK(a2) = 17.7, and pK(a3) = 28.6) compare unfavorably with experimental values of pK(a1) = 2.3, pK(a2) = 7. pK(a3) = 13, by the linear fit: 0.42907 x pK(a) - 0.23, the predicted pK(a)s become pK(a1) = 1.7, pK(a2) = 7.3, and pK(a3) = 12.1, which correspond well. The H3PO4 calculation shows a similar trend; pK(a1) = 4.9, pK(a2) = 19.9. and pK(a3) = 29.8 becomes pK(a1) = 1.9, pK(a2) = 8.3, pK(a3) = 12.6. Experimentally, H3PO4 has pK(a1) = 2.2, pK(a2) = 7.1, and pK(a3) = 12.3. With the same extrapolation, we predict H2SO3 to have pK(a1) = 1.1 and pK(a2) = 8.1, which compare favorably with the experimental values of pK(a1) = 1.9 and pK(a2) = 7.2.