In order to determine quantitatively the free radical content and its changes affected by additives using spin trapping under in vivo conditions, an approach is suggested carrying out experiments in a completely mixed open system (CMOS). Measurements have been carried out for a chemical oxidation process as a model system, and analysis of products and of the spin trap was extended by kinetic ESR spectrometry of the spin adducts. Since in a CMOS differential equations of accumulation of all species can be transformed into algebraic expressions using available rate constants for the formation of the spin adducts, corresponding concentrations of free radicals have been calculated. In addition, it has been established that triplet excited photosensitizers have a double effect: increasing the rate of initiation by decomposing hydroperoxide-type compounds and inhibiting the overall process by interactions with free radicals. Results indicate that by changing the "reaction vessel" the method can be applied for ex vivo and in vivo systems.