This paper covers a detailed analysis of the coordination changes taking place at the active sites in both Cu and Ni reconstituted hemoglobin as a function of pH. These experiments provide insight into how proteins are held in their native configuration. The EPR results of CuHb reveal that the species formed in extreme acidic condition were different from those formed at extreme basic condition. At pH 3 we see an isotropic spectrum characteristic of 4-coordinated species, while at pH 12 there is an indication of equilibrium between mixtures of species. Further support for the above coordination changes is obtained from FT-Raman of NiHb at different pH conditions. At pH 3 all the 5-coordination marker bands are lost and there is a shift in the 4-coordination marker band, while at pH 12 both 4- and 5-coordination marker bands are still seen with slight shift in their positions. In addition to this, we could see a new peak at 1633 cm-1. The coordination changes as a function of pH could be seen for both CuHb and NiHb using UV-visible spectroscopic techniques. Copyright © 2003 Society of Porphyrins & Phthalocyanines.