Singlet oxygen arising from the thermal decomposition of the water-soluble endoperoxide of 3,3'-( 1,4-naphthy1idene)dipropionate (NDP02) or from hypochlorite/H202 was detected. Direct measurements of light emission due to the chemiluminescent transition of O2 ('A,) to the triplet ground state was monitored (a) by monomol emission in the near-infrared (1270 nm) with a liquid nitrogen cooled germanium diode and (b) by dimol emission in the visible spectral region (634 and 703 nm) with a red-sensitive photomultiplier and photon counting. Chemical trapping of IO2 generated by N D P 0 2 or by hypochlorite/H202 was used for quantification with the anthracene-9,lO-diyldiethyl disulfate (EAS), yielding the endoperoxide (EAS02) as the specific oxidation product. The p value for EAS02 was calculated to be 7.7 X M when IO2 was generated by N D P 0 2 or hypochlorite/H202, respectively. Due to the enhancement of the lifetime of IO2, the / 3 values were lower in D 2 0 . The yields of '02 were measured, showing that one-half of the O2 liberated by thermolysis of N D P 0 2 was in the singlet state, whereas the yield for hypochlorite/H202 was near unity. The addition of 1,4-diazabicycl0[2.2.2]octane (DABCO) increased dimol emission with a concomitant decrease of monomol light emission, but left the yield of EAS02 unchanged. Similar results were obtained with hypochlorite/H202. A calibration of the photoemission was performed, based on the IO2 yield of N D P 0 2 thermodissociation. M and 8 X (4) Aubry, J. M.