To clarify the role of diazocarbonyl excited states in Wolff rearrangement chemistry, time-resolved infrared (TRIR) spectroscopy has been used to study methyl 2-diazo-(2-naphthyl) acetate (1) and the subsequently produced 2-naphthyl(carbomethoxy)carbene (2). TRIR analysis of the growth rate of the ketene rearrangement product following laser excitation of diazoester 1 demonstrates that in this case ketene is formed almost exclusively from the carbene. The absence of reaction from the diazoester excited state is likely due to the highly preferred anti relationship between the diazo and carbonyl groups in 1. In addition, the detection of IR bands from both the singlet and triplet states of spin-equilibrated carbene 2 has allowed a direct experimental estimate of the singlet/triplet energy gap in solution at ambient temperature. This work represents the first TRIR detection and study of a carbene intermediate and demonstrates the potential value of TRIR spectroscopy in structural and mechanistic aspects of carbene chemistry.