Abstract Resonance Raman (RR) and Fourier transform infrared (FTIR) spectra at 12K have been obtained for Pseudomonas aeruginosa azurin, spinach plastocyanin, stellacyanin, and tree laccase. The temperature dependence of the azurin, plastocyanin, and stellacyanin spectra have been recorded as have the RR excitation profiles at 12 K. Room temperature RR spectra have been obtained for azurins from Alcaligenes fecalis, Alcaligenes sp., Bortadella pretussis and Bortadella bronchiseptica; bean plastocyanin; fungal laccase, human ceruloplasmin; and zucchini squash ascorbate squash ascorbate oxidase. Isotope studies employing 63Cu/ 65Cu and H/D substitution have been performed on the azurins from Ps. aeruginosa, Alc. fecalis, and Alc. sp. Principal conclusion include the following; The intense RR modes near 400 cm −1 include internal ligand deformations and the Cu-S(cys) stretch, rather than the Cu-S(cys) stretch and Cu-N(his-Im) stretches as previously supposed. The Cu-N(his-Im) stretches are assignable to the ubiquitous feature near 265 cm −1,consistent with the frequencies of similar motions in other proteins and in model complexes. Spinach plastocyanin exhibits a frequency shift of 14 cm −1 in its cysteine CS stretching frequency ( ca. 750 cm −1) upon freezing of the protein solution, suggesting that extra-protein forces (e.g.,solvent structure, crystallization, or substrate binding.) can influence the conformation of the active site. Above the freezing point of the solvent the plastocyanin RR modes are unusually broad, suggesting either extremely facile due to thermally accessible conformations in the high-temperature form of the active site. No selective enhancement of either the strong or weak RR modes is observed in the S(cys)→ Cu charge transfer excitation profiles of azurin, plastocyanin, or stellacyanin at 12 K. The azurin species other than Ps. aeruginosa exhibit an ‘extra» strong RR peak near 400 cm −1 which is, however, seen to be related to an unresolved shoulder in the 12 K Ps. aeruginosa spectrum. It is therefore unnecessary to invoke higher coordination numbers than four for copper to explain the RR spectra of the azurins other than Ps. aeruginosa. RR peaks appear in the azurin spectrum below 200 K which may be due to methionine C-S stretching modes and Cu-S(met) stretch or methionine C-S-C angle bend. If these low-temperature features are indeed due to such motions,methionine must be closely coordinated to copper in azurin at low temperature. The RR spectra are consistent with a monotonic relationship between the force constant of the Cu-S(cys)bond and the energy of the ligand-field transitions of the various proteins. The isotope studies lead to tentative identification of the RR modes which contain significant contributions from M-L stretches and internal histidine motions. The results provide an initial basis for reliable structural interpretation of the RR spectra of the blue copper proteins.