Splittings related to the stretching vibration of bound dioxygen in hemoproteins have been detected by resonance Raman spectroscopy. With excitation at 406.7 nm we observe three isotope-sensitive lines in oxycobaltmyoglobin (oxyCoMb) [or in oxycobalthemoglobin A (oxyCoHbA)] at 1103 (1107), 1137 (1137), and 1153 (1152) cm-1, of which the most intense one appears at 1137 cm-1. The first two frequencies arise from resonance interaction between a v(O--O) mode at approximately 1122 cm-1 and an accidentally degenerate porphyrin ring mode at 1123 (1121) cm-1, whereas the third one represents an "unperturbed" v(O--O) vibration from a different species. These two v(O--O) modes at approximately 1122 and approximately 1153 cm-1 shift to approximately 1066 and approximately 1096 cm-1, respectively, upon 16O2 leads to 18O2 substitution. The same resonance interaction may also occur in oxyFeMb (probably also in oxyFeHb(a), because it exhibits an intensity increase at 1125 cm-1 upon 16O2 leads to 18O2 substitution, although the v(O--O) vibrations have not been observed directly. Concomitant enhancement is observed in the v(Co--O) vibration at 539 (537( cm-1, which is considerably lower than the v(Fe--O) frequency at approximately 570 cm-1 in oxyFeMb and oxyFeHbA. The Co--O bond is longer and weaker than the Fe--O bond. Enhancement of both v(O--O) and v(Co--O) indicates the existence of a charge-transfer transition underlying the Soret band, which may be assigned as pi*(pi g*O2/xz) leads to sigma*(dz2Co/pi g*). The presence of two v(O--O) vibrations (at approximately 1122 and approximately 1152 cm-1) but only one v(Co--O) mode at approximately 538 cm-1) means that the two species in oxyCoMB or oxyCoHbA have the same Co--O bond lengths but different O--O bond lengths. The bound dioxygen in a bent end-on configuration may have two allowed orientations, which differ in the extent of sp2(N epsilon) leads to pi*(O2) donation from distal histidine.