Triplet photosensitizers can be used for a variety of applications, including photocatalysis, OLEDs, and photodynamic therapy. Excited triplet states can be quenched by triplet oxygen to make singlet oxygen. Often the singlet oxygen quantum yield (Φ▵) is used as a lower approximation for the triplet yield. Unpredictable effects of even minor structural changes can drastically alter the Φ▵ and complicate the design of new triplet photosensitizers. The most common strategy to increase Φ▵ is to incorporate heavy atoms, promoting the “heavy atom effect”. However, the position and the identity of the heavy atom greatly influences the Φ▵. We have created a predictive model that correlates calculated natural atomic orbital composition of the heavy atom(s) contributing to the frontier molecule orbitals of a photosensitizer with the experimental Φ▵. The model, derived from several fluorescein derivatives, provides a calculated Φ▵ in agreement with the experimental values for a variety of well-known photosensitizers, including rhodamine dyes, fluorescein derivatives, and octahedral metal complexes.