The relaxation of vibrationally highly excited NCNO has been studied using the technique of time-resolved infrared fluorescence (TR-IRF). A pulsed dye laser operating at 600 nm, or in some experiments at 640 nm, excited NCNO to its S1 state whence radiationless transitions transfer the molecules to high vibrational levels of the S0 ground state below the dissociation limit. IRF was observed at ca. 4.6 μm, corresponding to fundamentals in the CN stretching vibration of NCNO. Values of 《ΔE》, the mean energy removed per collision with the diluent gas (Ar, N2 or He) were derived as a function of 《E》, the mean internal energy content of the excited molecules. The variation of 《ΔE》 with 《E》was approximately linear and, at 《E》 = 10000 cm−1,《ΔE》= 0.44 ± 0.03 cm−1 for Ar, 《ΔE》 = 0.62 ± 0.04 cm−1 for N2 and 《ΔE》 = 0.57 ± 0.04 cm−1 for He.