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Reverse intersystem crossing (RISC), the uphill spin-flip process from triplet to singlet excited states, plays a key role in a wide range of photochemical applications. Understanding and predicting the kinetics of such process in vastly different molecular structures would best lead to rational design of new materials. Here, we demonstrate a theoretical expression that successfully reproduces experimental RISC rate constants ranging over five orders of magnitude in twenty different molecules. We show that the spin flip occurs across the singlet–triplet crossing seam involving a higher-lying triplet excited state, where the semi-classical Marcus parabola is no longer valid. The present model explains counterintuitive substitution effects of bromine on the RISC rate constants in newly synthesized molecules, proving as predictive tool for materials design.