In this paper we advance a theory of nonadiabatic molecular group transfer processes in biological systems, which can be described in terms of a radiationless transition between vibronic levels corresponding to two distinct electronic configurations. The resulting multiphonon rate expression exhibits a continuous variation from a temperature-independent nuclear tunnelling rate at low temperatures to an activated rate at high temperatures. The theory is applied for the study of the recombination reaction between CO and hemoglobin (CO/Hb) in the temperature range 2-100 K. This process is accompanied by an electronic spin change of the system, is induced by weak second-order spin-orbit coupling, and involves large nuclear changes, whereupon the nonadiabatic multiphonon treatment is applicable. The CO/Hb recombination rate is expressed in terms of a product of a second-order spin-orbit electronic coupling term and a thermally averaged nuclear Franck-Condon vibrational overlap term. The experimental temperature dependence of the CO/Hb recombination can adequately be accounted for in terms of our theory, provided that the shift in the iron equilibrium configuration between the “free” and “bound” states is 0.4-0.5 Å, the characteristic frequency of the motion of the iron, coupled to the deformation mode of the heme group is ~100 cm-1, the upper limit for the energy change involved in the exoergic process is ~—0.05 to —0.1 eV, and the second-order spin-orbit coupling term is 0.1-1 cm-1. These nuclear and electronic parameters concur with the available information concerning structural and spectroscopic data for hemoglobin and related compounds.