The system P 2O 5 (vapor), SiO 2 (thin film) and single-crystal silicon has been investigated by solid-state-diffusion techniques. The p-type silicon underneath the silicon oxide film was used as a “phosphorus detector”. A rapid and complete chemical reaction apparently takes place between the SiO 2 and the diffusant, phosphorus or phosphorus oxide, forming a glass (or compound) of unknown composition P x Si y O z at the glass/silicon interface. A sharp boundary is found between the glass and unreacted SiO 2. The results indicate that the growth of the glass is limited by the diffusion of the phosphorus species in the glass with very little or no diffusant left to diffuse in the unreacted silicon dioxide after the complete reaction at the glass/silicon dioxide interface. The growth of this compound or glass follows the parabolic law and is experimentally given by x m 2 t m = 1.7 × 10 −7 exp( −l.46 kT m ) cm 2/sec or x m √ (t m) = 250 exp(− l.46 2kT m ) μ √ ( hr) over the temperature range 900–1250°C. Here x m is the thickness of the glass, t m the diffusion time and T m the diffus ion temperature (°K). The subscript m denotes the condition of complete masking of the silicon oxide against phosphorus, i.e. no n− p junction is formed. Diffusion experiments in which the silicon oxide film failed to mask or only partially masked against P 2O 5, i.e. x 0 2 td was smaller than the value given for x m 2 t m , where x 0 is the original oxide thickness and t d is the diffusion time, were also performed and could be interpreted by using a two-boundary diffusion model. In this case, a species of phosphorus or phosphorus oxide apparently diffuses through the glass with a diffusion coefficient, D 1, followed by the diffusion of phosphorus in the p-type silicon with a higher diffusion coefficient, D 2, to form a n− pjunction underneath the glass. The segregation coefficient of phosphorus at the boundary between the glass and the silicon is apparently small. The transition region between complete masking and partial masking is well defined and occurs at a silicon oxide layer thickness < 500 Å. Such a rapid change represents the transition between the models discussed above : the model for the growth law of the glass in silicon oxide and the two-boundary diffusion model for partial masking.