In ozone reactions, singlet dioxygen [O2(1Δg)] is formed when ozone reacts by O-atom transfer. O2(1Δg) yields were determined for more than 50 compounds using as reference the reaction of hydrogen peroxide with hypochlorite. Close to 100% yields were found in the reaction of O3 with sulfides, disulfides, methanesulfinic acid, and nitrite. In accordance with this, the only products are: methionine sulfoxide, methanesulfonic acid, and nitrate for the reaction of O3 with methionine, methanesulfinic acid, and nitrite, respectively. In the case of aliphatic tertiary amines (trimethylamine, triethylamine, and DABCO), the O2(1Δg) yields range between 70 and 90%, the aminoxide being the other major product. With EDTA and nitrilotriacetic acid (NTA), the O2(1Δg) yield is around 20%. The interpretation of the data with DABCO required the determination of the quenching constant of O2(1Δg) by this amine, kq = 1.8 × 109 dm3 mol−1 s−1 in H2O and D2O, two orders of magnitude lower than previously reported. Aromatic tertiary amines give even lower O2(1Δg) yields [N,N-dimethylaniline (7%), N,N,N′,N′-tetramethylphenylenediamine (9%)]. Substantial amounts of O2(1Δg) are also formed with the DNA model compounds, 2′-deoxyguanosine (40%) and 2′-deoxyadenosine (15%, in the presence of tert-butyl alcohol as ˙OH scavenger). The pyrimidine nucleobases only yield O2(1Δg) when deprotonated at N(1). O2(1Δg) formation is also observed with hydrogen sulfide (15%), azide (17%), bromide (56%), iodide (12%), and cyanide ions (20%). The O2(1Δg) yield from the reaction of O3 with phenols and phenolates is typically around 20%, but may rise closer to 50% in the case of pentachloro- and pentabromophenolate. Low O2(1Δg) yields are found with unsaturated acids such as dihydroxyfumarate (6%), muconate (2%), and acetylenedicarboxylate (15%). With saturated compounds, also, no O2(1Δg) (e.g. with propan-2-ol, acetaldehyde, acetaldehyde dimethylacetal and glyoxal) or very little O2(1Δg) (formic acid; 6%, at high formate concentrations) was detected. As shown with some examples, knowledge of the O2(1Δg) yield (in combination with that of other products) is a prerequisite for the elucidation of the mechanisms of O3 reactions in aqueous solutions.