Electron-transfer (ET) and atom- or molecule-transfer (AT) processes play a central role in a variety of biological systems. Notable examples are the primary events of photosynthesis,l the primary photochemical processes in some visual pigments? and the reversible uptake of oxygen and carbon monoxide by myoglobin and hemoglobin? The dynamics of all these processes are characterized by some general features exhibiting an Arrhenius-type temperature dependence of the unimolecular rate constant W at high temperatures, while at low temperatures W does not vanish but rather remains finite and temperature independent. The constant low-temperature rate reflects zero-point energy effects4 and manifests a nuclear tunneling phenomena.6 The modern theory of nonadiabatic multiphonon radiationless processes was recently successfully applied for the elucidation of the general features of a variety of such nonadiabatic electron-transfefl and group-transfer' processes, which can be explored from a unified point of view. It is of some methodological and practical importance to determine the transition temperature To from low-temperature nuclear tunneling to the high-temperature activated region without dwelling on elaborate numerical calculations. expressed the low-temperature rate in terms of the Gamow tunneling formula9 W = voexp(-6(pEA)1/2d/ h), where vo is a characteristic frequency, 6 = (2)lI2 is a numerical constant, p is a characteristic nuclear mass, EA is the barrier height, while d is the barrier width. Comparing the low-temperature rate with the activated rate W a exp(-EA/kBT) and disregarding preexponential terms, Goldanskii concluded that the two rates are equal at the temperature kBTo = hEA1/2/6p1/2d. Goldanskii's result rests on firm theoretical grounds in view of the equivalence between the Gamow exponential tunneling factor and the Franck-Condon vibrational nuclear overlap factor.1° It would be useful to provide an alternative and more transparent expression for kBTo on the basis of the modern theory of nonadiabatic multiphonon processes, which is the subject matter of the present note. The results are useful for the analysis of the primary ET events in photosynthesis. We consider a nonadiabatic multiphonon ET or AT process occurring between two nuclear potential surfaces which are both characterized by the frequency o; the horizontal (reduced) displacement of their minima is A, while the vertical displacement between the minima representing the energy gap is M. The electron vibration coupling strength is S = A2/2, while the oo~~-~~~~iaot~oa ~-~~~o$o1 .ooto