The subunit dissociation of human Hb A by the aliphatic acid salts at neutral pH was investigated by light-scattering molecular-weight measurements at 630 nm. Dissociation of Hb tetramers to .alpha..beta. dimers is observed in essentially all experiments at low to intermediate levels of salt concentrations below the denaturation transitions. The effectiveness of the salts as subunit dissociating agent, reflected by the slopes, s, of the plots of .DELTA.GD.degree., the standard free energy of dissociation, vs. [D], the salt concentration, increases with increasing alkyl chain length or hydrocarbon content of the salt. Estimates of the apparent number of amino acid sites at the areas of contact per .alpha..beta. dimer formed, N'', based on the slopes of the higher members of the series was obtained using the equation, .DELTA.GD.degree. = .DELTA.GD,w.degree. - 2N''RTKB[D]. Independent estimates of the binding constant, KB, required for these calculations were based on free-energy transfer data of hydrophobic amino acid alkyl groups and protein denaturation data. Estimates of N'' obtained with the more reliable data of the higher members of salt series are in the ranges of 19 and 27 amino acid groups, shown by the X-ray crystallographic structure of horse and human Hb of Perutz and Fermi for the smaller .alpha.1.beta.2 contact areas in the tetrameric structure. The lower estimates than 27 based on these dissociation of human Hb suggest that several of the amino acid residues in the contact areas of the subunits are partially exposed to solvent. The increasing effectiveness of the higher members of acid salt series and the alkylureas suggests that hydrophobic interactions are an important source of stabilization of the tetrameric structure of Hb.