Abstract Protein unfolding of eight different phycocyanins was investigated utilizing circular dichroism and visible spectra. The phycocyanin samples were extracted from algae that are normally found in vastly different environments, and are classified as mesophilic, thennophilic, halophilic and psychrophilic. The ability of these proteins to resist the denaturant urea is in the order of thennophile > mesophile, halophile > psychrophile. Based on a two-state approximation the apparent free energies of protein unfolding at zero urea denaturant concentration, Δ app H 2O were found to range from 2.4 to 8.8 kcal mole for the eight phycocyanins at pH 6 and 25° C. The proteins from the thermophile are generally more stable than those from the mesophile. An extra stability of the halophile is believed due to the specific interaction of the proteins and the ions in solution. A correction for Δ app H 2O due to minor amino acid differences reveals that the stability and the structural properties of these proteins are primarily affected by this minor difference in amino acid compositions.