A technique for studying the spin evolution of radical ion pairs is proposed, which is based on switching off the magnetic field with a certain time delay after the ionizing radiation pulse giving rise to the pairs. The spin evolution of radical pairs upon field switching is described by a quasiclassical model. The modeling of magnetic field effect curves, measured for n-decane solutions of deuterated and protonated paraterphenyl, provides fair agreement between theory and experiment. (C) 2008 Elsevier B.V. All rights reserved.