The three O 2 emission systems that emit in the ultraviolet, Herzberg I ( A3∑ u+- X3∑ g-), Chamberlain ( A' 3∆u- a1∆g) and Herzberg II ( c1∑ u-- X3∑ g-), have been studied. Measuremen Herzberg I and Chamberlain emission profiles have been analysed by considering different excitation mechanisms for each state. Photochemical schemes are proposed to explain the measured emission profiles and the relative intensities of the emissions coming from these O 2 excited states. This photochemical scenario is shown to be compatible with the theoretical results and with the rest of the molecular oxygen emissions. The altitude integrated vibrational distributions of the Herzberg I, Herzberg II and Chamberlain bands detected in the nightglow are reproduced. The predicted altitude profiles of the individual vibrational levels can reproduce the experimental results available at present.