The effect of nitrogen concentration on boron diffusion in silicon oxides (oxynitride) used for metal-oxide-semiconductor structures was investigated. The oxynitrides, which were formed by oxidizing thin, thermally grown nitrides, contained uniform amounts of nitrogen. The boron diffusion coefficients in the oxynitrides were determined, and experimental and simulated results were compared. The diffusion coefficients have an Arrhenius relationship to each concentration of nitrogen, and are smaller in higher nitrogen concentrations. A 25% concentration of nitrogen exhibited an oxynitride diffusion coefficient at least two orders smaller than that of SiO2,. The higher the nitrogen concentration was, the larger the activation energy was. The diffusion coefficient data is useful for evaluating the boron penetration of various types of oxynitrides, including nitrided oxides. An empirical diffusion model is proposed in order to explain the experimental data qualitatively.