Electron spin polarization (CIDEP) in free radicals created by reaction of a photo-generated triplet is transmitted, if reaction occurs within their spinlattice relaxation time, along a reaction pathway to secondary radicals. The time-dependence of the E.S.R. signal from the secondary species, obtained at a specific magnetic field, is analysed for periods of time of the order of its relaxation time after the photolysis flash that creates the triplet. It is shown that the relaxation times of the secondary radical can be obtained, thereby greatly increasing the variety of radicals whose relaxation may be studied by rapid-response E.S.R. methods. The relative rates of reaction of a given primary radical with diverse substrates are obtained from measurements of the polarization observed in the secondary species in various solutions.