Abstract Thermal decomposition of 1,4-dimethylnaphthalene endoperoxide (DMNE) as a source of singlet oxygen was used to measure chemical rate constants, k R , for reactions between singlet oxygen, O 2 ( 1 Δ g ), and various substrates. Time resolved O 2 ( 1 Δ g ) IR luminescence detection and steady-state experiments were used to monitor the decomposition of the endoperoxide and the rate of singlet oxygen production. Only 25% of oxygen from thermal decomposition of DMNE, in acetonitrile at 20 °C, is detected as O 2 ( 1 Δ g ). Values of k R for reactions of O 2 ( 1 Δ g ) with 1,4-diphenylisobenzofurane (DPBF) and rubrene measured by this method are similar to values obtained by photosensitization. Values of k R for the chemical reaction of O 2 ( 1 Δ g ) with the antiinflammatory drugs piroxicam and tenoxicam, of (6.1±0.4)×10 6 and (1.6±0.2)±10 7 M −1 s −1 , respectively, are close to those for the total singlet oxygen deactivation rate. Thermal decomposition of aromatic endoperoxides is a convenient source of singlet oxygen for measurements of rate constants in reactions of O 2 ( 1 Δ g ) where photosensitization cannot be employed. However, experimental conditions and approaches involved determine the method’s limitations and applicability in a given system.