The interaction of oxygen with CdSe surfaces has been studied with thin films of CdSe evaporated in ultra high vacuum in the temperature range 0-360°C and pressure range 10-4-10mm Hg. The effect was monitored via conductivity measurements, carried out in situ. When oxygen is introduced, the initially "clean" film undergoes an instantaneous irreversible chemisorption followed by a slow uptake which is also irreversible in the studied temperature range. This interaction brings about a 3-5 orders of magnitude decrease in the conductivity of the n-type film, i.e. oxygen exhibits acceptor-like properties. The rate of the slow irreversible oxygen uptake can be expressed as dN dt = kP 1 4o2 exp( - bN) and the activation energy for the process has been measured. Donor type weakly adsorbed oxygen was also found to be present on CdSe films which could be reversibly removed from the surface; this reaction was also accompanied by charge transfer and was strongly pressure dependent. The heat of reaction for this process is 12 kcal mole. The boundary layer theory was evoked to explain the kinetics of the slow uptake of the acceptor-like oxygen. Attempts have been made to explain the mechanism of the different surface reactions. © 1963.