The effect of the foreign gases Hydrogen, Helium and Deuterium on the lover explosion limit of P4-02 mixtures has been studied, and the values for the inert gas effect determined in each case. Comparison of these values showed conclusively that hydrogen ha: a greater inert gas effect than is predicted by theory, A partial explanation of this may lie in the assumptions made as to the nature and physical constants of the chain carriers. The absorption spectrum of phosphorus vapour was obtained for different pressures of phosphorus vapour. Definite absorption was seen to take place in the region of and below the mercury line at 2537A ?. Illumination of P4 -02 mixtures at pressures below the lower explosion limit produced two effects, viz. (a) a displacement of the lower limit towards smaller pressures (b) a reaction between the P4 and 02 molecules below the lower limit. Further experimental work was carried out on the displacement of the lower limit in order to test the statement that the explosion limits in a chain reaction are independent of the rate of initiation of the chains. loo During this work it was shown that the displacement is not permanent but slowly decays on allowing the reaction mixtures to stand before proceeding to explosion. A period of five to six minutes is necessary before normal conditions are regained. Illumination of P4 vapour alone resulted in a displacement of the lower limit of only half the value of that obtained when oxygen was present, although the slopes of the corresponding decay curves were similar. Moreover the displacement was the same for different pressures of oxygen, although it was obviously effected by a change in the condition of the walls. All the evidence obtained which was based on the transfer experiments, showed that the displacement of the lower limit was a wall effect and was not due to the production of active centres in the gas. This wall effect was due in the first case to the formation of P2 molecules from the P4 molecules. Illumination of phosphorus- oxygen mixtures above the upper explosion limit resulted in a displacement of this limit towards higher pressures. It was suggested that the explosion limit found under normal conditions is low owing to the fact that the rate of starting of the chains is too slow for the relationship predicted by chain theory to hold. The stable reaction between P4 -02 molecules initiated by the ultraviolet light was shown to be a chain reaction, the kinetics of which was studied and the rate of reaction was found to be given by =_ d[02]/dt = [P4][02]2 x I x constant. The effect of inert gases on the reaction rate was studied and it was shown that the velocity constant was independent of the concentration of inert gas present. Phosphorus vapour on exposure to ultraviolet light was shown to be converted into red phosphorus. The rate of transformation was measured and the reaction found to be approximately unimolecular.