Absorption spectra of various cytochrome preparations were measured in the visible region over a wide range of temperatures from that of liquid helium (4.2°K) to room temperature (300°K), employing the double Dewar's vessel system of Hagihara and Iizuka. The preparations used were purified beef-heart cytochromes a (a3), b, and C1, crystalline pigeon breast-muscle cytochrome c and crystalline yeast (Candida krusei) cytochrome c, all in the reduced form, and crystalline pigeon breast-muscle cytochrome c in the oxidized form. Remarkable sharpening of each peak in the spectra of the reduced cytochromes, especially of the α-peak, was observed when the temperature of the samples was lowered. In the case of animal cytochromes c and C1, the α-peak separated into two distinct peaks (α1 and α2) at low temperatures as a result of this sharpening. A wavelength shift of the α-peak occurred toward the blue end (by 3–5nm) when the temperature was lowered from 300°K to about 10°K, except in the case of cytochromes c and c1 whose α-peaks were not or were only slightly shifted by peak splitting. Remarkable intensification of extinction was also observed with lowering of the temperature, and this, together with the above sharpening, can be very useful for identification of each cytochrome in mixtures. Comparing the spectra of cytochromes c1and b, whose structures are not known, with those of the well-studied cytochromes c and b5, the electronic state and heme-environment of cytochromes c1 and b are discussed.