The electron paramagnetic resonance (EPR) studies on the acid-base equilibria of organic molecules in the lowest excited triplet (T₁) states are reviewed. The combined method of the time-resolved EPR technique and the stretched poly(vinyl alcohol)-film technique is useful for studying the acid-base equilibria of short-lived and very weakly phosphorescent T₁ molecules, where the steady-state EPR and optically detected magnetic resonance techniques are not applicable. By applying a microwave magnetic field parallel to an external magnetic field, the proton hyperfine structures in ΔMs = ±2 transitions have been observed for the T₁ states of organic molecules in glassy matrices at 77 K. The parallel-polarized EPR method is useful for studying the anisotropic hyperfine structures of molecules under acid-base equilibria, where a single-crystal method is not applicable. The effects of protonation and deprotonation on the spin densities, zero-field splittings and anisotropic S₁ → T₁ intersystem crossing rates for the three T₁ sublevels are discussed.