Abstract— We report the detection of a weak near-infrared light emission originating from 8 nM singlet molecular oxygen (1O2) produced in a mixture of 1 mM hypochlorite (OC1-) and 8 nM hydrogen peroxide (H2O2). The measurements were made with a highly sensitive detection system for ultraweak light emission in the 1.0-1.5 μm wavelength region. The emission intensity exhibited linear dependence for H2O2 concentrations in the range of 8-670 nM. The mixture containing a lower concentration (33 μM) of OCl- pseudocontinuously emitted near-infrared light for 5 s. The rate constant for 1O2 production obtained from the kinetic analysis agrees with that previously reported. Our results demonstrate the possibility of measuring very low concentrations of 1O2 in a OCi-/H2O2 mixture as well as 1O2 production in intact living systems.