The diffusion mechanism of phosphorus into thermally grown SiO2 films from phosphorus vapor has been investigated using secondary ion mass spectrometry, wet-chemical spectrophotometric measurements, and x-ray photoelectron spectroscopy. The diffusion coefficients of phosphorus in SiO2 films were determined in the temperature range 1273 to 1373 K : D-p/cm(2) s(-1) = 3.79 x 10(-9) exp (-221000/RT) where R is 8.31 J K-1 mol(-1). The solubilities of phosphorus in SiO2 films at 1373 K ranged between 3.0 x 10(20) and 2.0 x 10(22) cm(-3) in the P-2 partial pressure range 0.001 to 0.22 atm and were approximately proportional to the first power of the P-2 partial pressure. The x-ray photoelectron spectra revealed the presence of phosphorous pentoxide and atomic silicon in SiO2 films which underwent phosphorus diffusion. The mechanism for phosphorus diffusion in SiO2 films is as follows : after dissolving into the interstitial site of SiO2 in the form of P-2, phosphorus is incorporated into the network of SiO2 by exchanging its site with silicon in the lattice site and diffuses through the silicon lattice sites.