The reaction mechanism for [2+2] cycloaddition of chlorosulfonyl isocyanate (CSI) with alkenes is presented. Electron-deficient alkenes react by a concerted pathway while most alkenes, and especially electron-rich alkenes, react by a single electron transfer (SET) pathway to give a 1,4-diradical intermediate. NMR line-broadening studies show that the triplet 1,4-diradical intermediate can be converted to the singlet form at lower temperatures. UV data give evidence for a pre-equilibrium charge transfer complex that is confirmed by kinetic studies. That is, the reaction is more efficient at a lower temperature where the pre-equilibrium favors the charge transfer complex compared to a slightly higher temperature where the pre-equilibrium favors the dissociated reagents. These mechanistic findings were incorporated to enhance the synthetic usefulness of CSI [2+2] cycloaddition reactions.