We have generated radical pairs (RPs) in SDS micellar medium by electron transfer from the ground state of biphenyl (BP) to various alkyl-substituted phenyl pyrilium cations (PP+) and corresponding thio-phenyl pyrilium cations (SPP+) and have compared the dynamics of RP recombination in magnetic fields varying from 0 to 5 T. A comparison of PP./BP.+ decay curves with SPP./BP.+ decay curves leads to the conclusion that both (1)RPs and (3)RPs are produced in the former case, the temporal behavior at early times being dominated by field-insensitive (RP)-R-1 and at later times by highly field-sensitive (3)RPs. In the case of the SPP./BP.+ system, on the other hand, (3)RPs dominate the decay throughout but the field sensitivity is reduced. The large MFE in O-analogues is presumably caused by reduction of micellar escape rate while the reduced MFE for the S-analogues has been ascribed to spin-orbit coupling (SOC).