Magnetic field effect (MFE) on the radical pairs (RPs) generated by the photoexcitation of the phenyl pyrylium ion (PP+) in the presence of the electron donor biphenyl has been investigated. A large effect was observed particularly for the BP/PP+ (1) case; the escape yield at 5 T was more than 20 times the zero-field value at a long time delay. The low-field variation of MFE conforms to the pattern expected for the isotropic HFC (hyperfine coupling) mechanism, and the high-Field variation conforms to that expected for the relaxation mechanism. The addition of salt causes saturation at a slightly lower field, presumably because of a change in the rotational correlation time (local motion) of PP. at the micellar surface. Introduction of a methyl group in the acceptor reduces the MFE considerably the reason for which is not immediately clear.