In the present Letter, the reaction operator of the main equation of spin chemistry has been modified on the basis of an exactly solvable model. The model contains both the reversible and irreversible transitions. For this model, expression for the reaction operator is more complex due to the expanded basis of spin states. The reaction operator of a phenomenological equation is a particular case of a more general approach and only in the limiting case of the completely irreversible recombination process, these operators coincide. (c) 2014 The Authors. Published by Elsevier B.V.