  Since the 1970s, spin-selective reactions of radical pairs have been modelled
theoretically by adding phenomenological rate equations to the quantum
mechanical equation of motion of the radical pair spin density matrix. Here,
using a quantum measurement approach, we derive an alternative set of rate
expressions which predict a faster decay of coherent superpositions of the
singlet and triplet radical pair states. The difference between the two
results, however, is not dramatic and would probably be difficult to
distinguish experimentally from decoherence arising from other sources.
