Using fast flash photolysis, we have measured the binding of CO to carboxymethylated cytochrome c and to heme c octapeptide as a function of temperature (5 degrees-350 degreesK) over an extended time range (100 ns(-1) ks). Experiments used a microsecond dye laser (lambda = 540 nm), and a mode-locked frequency-doubled Nd-glass laser (lambda = 530 nm). At low temperatures (5 degrees-120 degreesK) the rebinding exhibits two components. The slower component (I) is nonexponential in time and has an optical spectrum corresponding to rebiding from an S = 2, CO-free deoxy state. The fast component (I*) is exponential in time with a lifetime shorter than 10 mus and an optical spectrum different from the slow component. In myoglobin and the separated alpha and beta chains of hemoglobin, only process I is visible. The optical absorption spectrum of I* and its time dependence suggest that it may correspond to recombination from an excited state in which the iron has not yet moved out of the heme plane. The temperature dependences of both processes have been measured. Both occur via quantum mechanical tunneling at the lowest temperatures and via over-the-barrier motion at higher temperatures. © 1978, The Biophysical Society. All rights reserved.