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Singlet oxygen ( 1 O 2 ) is the first electronic excited state of molecular oxygen. Due to its non-radical and non-ionic character as well as its mild reactivity, 1 O 2 has a pivotal role in cell signaling processes at low concentration, yet it is cytotoxic at high concentrations. Quantifying the production of 1 O 2 , particularly in biological systems, is therefore essential for understanding and controlling its effects. 1 O 2 can be produced by chemical and biological reactions, yet its most common method of production is by photosensitization, whereby an initially photoexcited molecule transfers its acquired electronic energy to the dioxygen molecule. The efficiency of this process is characterized by the 1 O 2 production quantum yield, Φ Δ , which can be determined by directly monitoring its intrinsic weak near-infrared phosphorescence or indirectly by trapping it with a suitable acceptor, a process that can be monitored by common analytical techniques. Indirect methods are thus very popular, yet they may lead to severe errors if used incorrectly. Herein we describe the common aspects of indirect methods and propose a general step-by-step procedure for the determination of Φ Δ values. In addition, we identify the key experimental conditions that need to be controlled to obtain meaningful results.