The infrared spectrum of triplet excited duroquinone occluded in zeolite NaY has been recorded by time-resolved Fourier transform-infrared spectroscopy on the nano- and microsecond time scale. A commercial step-scan instrument whose capability was expanded to allow absorption spectroscopy at nanosecond resolution was employed for this study. O-18 isotope labeling allowed assignment of a strong absorption at 1542 cm(-1) to the CO stretching mode. The 100 cm(-1) red shift of this band relative to the CO stretch of the ground state establishes the pi pi* character of the lowest triplet state of duroquinone. This is consistent with the highly polar environment of the zeolite NaY cage. The lifetime of the excited triplet state is 20 mu s (-50 degrees C). This constitutes the first time-resolved infrared spectrum of a transient molecule in a zeolite.