An accurate expression for the recombination yield of geminate radical pairs (RPs) in low magnetic fields has been derived by a steady state approach to the stochastic Liouville equation. It is assumed that the relative motion of the radicals can be described as a free diffusion and that the spin mixing is caused solely by the hyperfine interaction of a single nuclear spin and the difference in Zeeman energy of the two radical partners. We introduce a new approximation that simplifies the calculations and the resulting expression significantly and which, unlike the popular level population description, preserves the important coherences. The accuracy of the explicit expression is illustrated by comparison with accurate numerical results. (C) 2002 Published by Elsevier Science B.V.