The dynamics of photochemical reactions exhibiting chemically induced electron spin polarization (CIDEP) are modeled by kinetic equations that include all relevant parameters, including reaction of the excited precursor as both a singlet and a triplet and spin exchange during radical encounters. Analysis of the acetone/2-propanol photolysis reveals spin exchange effects and provides further evidence that the anomalous net electron spin resonance (ESR) absorption of the 2-propanolyl radical is due to spin-lattice relaxation in the triplet acetone precursor. The weakening of this net absorption in acetone/triethylamine photolyses is due to the rapid reaction of photoexcited acetone with triethylamine resulting in reaction via both its singlet state, which cannot produce a net polarization, and its triplet state, which yields no triplet polarization and reacts before spin-lattice relaxation can produce full equilibrium polarization.