We studied boron diffusion in thin silicon oxides including pure SiO2 and oxynitride that are used for metal-oxide-semiconductor transistors. We measured the boron penetration using secondary ion mass spectroscopy. By comparing simulated and experimental results, we found that the boron diffusivity in pure SiO2, D-PO, is 3.96 X 10(-2) exp (-3.65 eV/kT) cm(2)/s, and that in oxynitride containing 4% nitrogen, D-NO, is 3.42 X 10(-2) exp (-3.75 eV/KT) cm(2)/s. We also obtained an analytical critical time model for the onset of boron penetration. Our model predicts that the critical time is proportional to reciprocal of diffusivity, and hence the time in oxynitride is about 2.5 times that in pure SiO2.