In this paper we analyze from a theoretical point of view the unusual isotope effect ascribed to the addition of protium and muonium hydrogen isotopes to pyrazine. By means of ab initio calculations the minima and the transition states corresponding to the addition of hydrogen to either a carbon or a nitrogen atom of the ring are localized. The, intrinsic reaction coordinate (IRC) potential energy curves are then obtained in order to calculate the transmission coefficients so that the rate constants for the different processes are obtained. It is found that both protium and muonium will preferentially react by adding to the carbon atom of pyrazine. This result does not explain the unusual isotope effect recently described. A new interpretation of the experimental results which does not require the above mentioned anomalous effect and agrees with the theoretical results is finally suggested.