The calculated energy barriers for H/D exchange in E:S, E:P and apo BSS indicate that the exchange from protium to deuterium in the glycyl radical occurs at an approximately 7-fold higher rate in E:S and E:P than the backward exchange of the deuterium to the protium. In the apoenzyme, we calculated the inversed situation, with a 4.3-fold slower rate of protium to deuterium exchange compared to the reversed process. However, if the rates of E:S and E:P and apo are compared we find very similar rates for E:S and E:P complexes but the calculated rates for the apoenzyme are more than five orders of magnitude slower. These findings are in qualitative agreement with the results of isotope exchange experiments monitored by EPR spectroscopy. If we assume that the time needed for preparation only allowed for partial exchange of deuterated glycyl radical with protium we would expect full exchange of protium by deuterium because of the higher rate in the case of E:S or E:P complexes. In the case of apo BSS the calculated rate was so low that any H/D exchange would not be observable in reasonable time.