A comparative denaturation of HbA and HbS in the R states using sodium n-dodecyl sulfate (SDS) was carried out at pH 7.20 in the presence and absence of Calcium (0-40 μM) and monitored by UV-Vis spectrophotometry in the range of 250-650 nm. In the HbS spectra, the calcium alone caused little or no perturbation of the aromatic region but caused a decrease in oxygen affinity when compared to the HbA. The combinations of [SDS] and [Ca] perturbed the HbS the most, relative to the individual spectra of the [SDS] and [Ca]. However, the presence of Ca appeared to diminish the adverse effects of the SDS on HbA. The denaturation pathway of the HbA involved mainly the formation of heme dimers and some ferryl heme species. For the HbS, heme monomers and a large amount of ferryl species were formed. It is suggested that the greater monomer species formed by the HbS denaturation pathway would result in both Fenton and enhanced enzymatic reactions, compared to the dimer. This could lead ultimately to the formation of ferryl radicals. Thus, at physiological pH for the HbS, the Ca-SDS interaction increases the tendency for protein denaturation in comparison to the HbA.