[en] The recombination/separation of the photochemically created singlet radical pair to the ground state and excited triplet products is studied. The spin-conversion in a pair is considered as a stochastic (incoherent) process, assuming that the recombination of both singlet and triplet radical pairs is contact. The quantum yields of recombination products are calculated for any initial separation of radicals in a pair. A special attention is paid to the dissipation of contact born pairs from both singlet and triplet states or via one of them. It is shown that the viscosity dependence of the product yields of the double-channel recombination differs qualitatively from the single-channel one whatever channel is open for the recombination. The validity range of incoherent description of spin-conversion is clarified by a straightforward comparison of the recombination efficiencies with 1those obtained within the Hamiltonian description of spin-conversion. In addition the role of the radical pair distribution over initial (starting) distance is discussed as well as contact approximation for recombination