An analytical study of the interaction of mixing, radiation attenuation, and chemical kinetics in isothermal photoreactors is presented. For a particular chain reaction mechanism in the presence of stationary state kinetics and low conversion, the conditions required for the existence of mixing effects are formally stated, and the direction of change of conversion and quantum yield resulting from the introduction of mixing is established. Calculated results are presented for monoenergetic, unidirectional sources. Factors considered include mode of chain termination, radiation attenuation law, photoreactor geometry, state of mixing, and reactor optical thickness. Chemical and mixing time scale considerations are discussed.