Hemoglobin plays an important role in transporting oxygen in human beings and other mammals. Hemoglobin is a tetrameric protein composed of two alpha and two beta subunits. The £\ and £] subunits are both necessary and the stoichiometric ratio of the two dislike subunits is critical for hemoglobin to perform its oxygen-carrying function properly. To better understand the coupling between the £\ and £] subunits and the subunit disassembly pathway, p-hydroxymercuri-benzoate (PMB) has been used to react with the cysteine residues in hemoglobin. The hemoglobin tetramer becomes unstable and disassembles into £\ and £] subunits when the cysteine sites are perturbed upon reacting with PMB. There are three kinds of cysteine residues, £]93, £\104 and £]112, in human hemoglobin. The reactivity of different cysteine residues with PMB and their reaction sequence have been studied via the Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). The resonance Raman spectroscopy has been used to investigate the structural changes of hemoglobin accompanying the PMB-modification under the oxygenated and deoxygenated conditions. At last, a hemoglobin subunit disassembly mechanism is proposed and the site-specific roles of cysteine residues in human hemoglobin are discussed in detail.