Vibrational relaxation times in pure oxygen have been measured over the temperature range 100°–200°C. An anomalously rapid decrease in the relaxation time is observed for temperatures above 150°C, confirming earlier conclusions based on a comparison of theory and experiment [J. G. Parker, J. Chem. Phys.41, 1600–1609 (1964)]. This decrease in the vibrational relaxation time may be interpreted as a transition in oxygen from a low‐temperature state to a high‐temperature state, although the exact nature of the physical mechanism underlying this transition remains obscure.