Time resolved near IR luminescence detection of singlet oxygen, O2(¹deltag), and steady-state photolysis experiments were performed to study in detail limitations and approaches involved when 9,10-dimethylanthracene (DMA) is used as actinometer to measure the chemical rate constants, kr, for the reaction between excited oxygen and a given substrate. Our results show that in solvents in which singlet oxygen lifetime is long, the actinometer opens an additional pathway to the singlet oxygen disappearance at a rate of similar magnitude to the decay rate constant of O2(¹deltag), kdelta. This reactive pathway decreases singlet oxygen concentration. In this case erroneous values of the chemical reaction constant, k rM, for the reaction between singlet oxygen and a given substrate M will be obtained. Additionally, we have found that not in all the solvents, can the total rate constant, kT DMA, for the reaction between singlet oxygen and 9,10-dimethylanthracene obtained from time resolved experiments be taken as the "reactive" rate constant, k rDMA, when DMA is employed as an actinometer. The chemical reaction constant, k rM, for the reaction between singlet oxygen and a given substrate M obtained in these conditions will be smaller than the true values. Then, to employ DMA as actinometer, kT DMA and k rDMA must be previously evaluated. If kT DMA and k rDMA values are very close, nearly ideal conditions to employ DMA as actinometer are fulfilled. Moreover, if kTDMA and krDMA differ in a greater extent, further corrections must be applied to improve k rM values.