The recombination of a singlet born radical pair subjected to spin conversion into an excited triplet product was studied a number of times with either the coherent (Hamiltonian) or incoherent (rate) model of spin transitions. The comparative study of these two approaches to systems of two interacting spin levels (in a high magnetic field) is presented. The recombination quantum yield is shown to be the same in both cases, except that the single parameter related to spin conversion is different. However, this difference does not affect significantly the diffusional dependence of the effect. (c) 2005 Elsevier B.V. All rights reserved.