Cs NH azetidine functionality, as well as a free "bis-homologated" oxetane carboxylic acid or a tetrahydropyranone heterocycle after oxetane ring-opening, depending on the conditions used for ester removal. An experimental comparison of the reactivity of different benzylic radicals revealed only oxetane and azetidine substrates to favor a productive Giese reaction pathway with the other benzylic linkers showing significantly lower yields and/or increased amounts of dimer side products. A computational investigation revealed the Giese addition of benzylic radicals into acrylate acceptors to be reversible in some cases, with less stable radicals shifting the equilibrium towards the coupled product. Furthermore, reduced spin density on the benzylic carbon was found to minimize formation of the dimer side product. Oxetane radicals, and by analogy azetidine radicals, are the only species that lie in the sweet spot of undergoing little degradation of the benzylic radical, whilst showing an exergonic Giese addition and minimal dimer formation. Experimental and computational studies are presented to explain the differing reaction outcome with certain acceptors, and to suggest the mechanism for the formation of side products. We envisage this protocol will encourage the use of 3-aryl-3carboxylic acid oxetanes and azetidines as convenient radical precursors in medicinal chemistry and expand the medicinal chemist's toolbox for the incorporation of 4-membered rings into drug-like compounds. The computational and experimental mechanistic investigations presented herein improve the general understanding of photoredox catalyzed reactions, particularly in Giese-type transformations and of strained ring and/or benzylic substrates. These fundamental studies will serve as a blueprint for the development of further methodologies that involve such substrates, and our insights will provide synthetic chemists with important factors to be considered during the optimization of such reactions.