The external heavy-atom effect, which is commonly used to enhance the triplet yield of photosensitizers, is applied here to allow the determination of accurate values of the parameter S_Δ that describes the efficacy with which the triplet reacts with O₂ to produce O₂ (¹Δ_g) (quantum yield Φ_Δ). The study of the effect of halogenated compound (Q) addition on the luminescence intensity of O₂ (¹Δ_g) at 1270 nm or on the rate of O₂ consumption for three porphyrin derivatives allows to get precise values of Φ_Δ/S_Δ and then of S_Δ. It is concluded that (i) the probability S_Δ is equal to unity for TPP, MgTPP and pheophytin a, (ii) the best balance between ease of use and result accuracy is provided by the time-resolved luminescence of O₂ (¹Δ_g) at 1270 nm technique and the determination of the kinetic parameters using a relationship involving relative change in fluorescence yields F/F_Q and in luminescence intensities of singlet oxygen L_Q/L, and (iii) TPP, a readily available sensitizer, is recommended as a reference compound with Φ_Δ = 0.80 in aerated benzene.