The applicability of phosphorescence spectroscopy is investigated for cure characterization of epoxy resin cured by an aromatic diamine. By using a time-resolved method to separate fluorescence from phosphorescence, the room-temperature phosphorescence (RTP) behavior of a commonly used curing agent, bis(p-aminophenyl) sulfone (DDS), is studied. RTP emission spectra with a maximum around 500 nm are obtained. Both RTP emission and excitation intensity increase very sharply with cure time due to the increase in the rigidity of the cured epoxy. They can be correlated to the extent of cure of epoxy, when properly calibrated. The triplet lifetime (tau(Tau)) obtained by monitoring RTP emission decay at 500 nm increases from zero to about 40 ms following cure at 160 degrees C for 180 min. Correlation curves of tau(Tau) with the extent of cure and the glass transition temperature are also obtained. Thus, tau(Tau) provides a way to estimate the cure extent without the need to calibrate the RTP emission intensity. While these RTP characteristics cannot be used for in-situ cure monitoring, they can be used as a sensitive cure estimate technique in lightly cured prepregs for composites or after a cure cycle.