The temperature dependence of the visible-near-infrared absorption spectra of Pseudomonas aeruginosa cytochrome c-551 aqueous solutions was studied in the temperature range 20-80 degrees C. For a neutral or acidic solution the absorbance at 695 nm decreases and that at 620 nm increases with increasing temperature, which possibly corresponds to the transition from the low-spin state to the high-spin state of the ferricytochrome c-551 heme at a higher temperature. For an alkaline solution the absorbances at 551 and 521 nm, which are the characteristic absorptions of ferrocytochrome c-551, increased with incresing pH value of the solution and with increasing temperature. It was speculated that the spectral change in alkaline solution resulted from the cleavage of the chemical bond between the heme iron and methionine-61 sulfur then probably followed by combination of OH- with the heme iron.