Thermally grown silicon dioxide thin films on silicon were nitrided in 1 atm at 1150°C for various times between 10 and 240 min. The films were between 12 and 163 nm thick. Chemical kinetics severely limited the incorporation of nitrogen into the film; only the thinnest films nitrided for times longer than 90 min attained a nitrogen content near 40 atomic percent nitrogen. Secondary ion mass spectrometry indicated that nitridation either lowered or maintained the average hydrogen concentration in the film. Infrared spectroscopy demonstrated that nitridation removed silanol groups from a film which had been grown in wet oxygen. In certain films, over 99% of the nitrogen in the film was incorporated into the tetrahedral network; less than 1% of all nitrogen in the film existed as silamine groups. Nitridation increased (i) the 1 MHz dielectric constant and (ii) the electron‐trapping ability of a film; these two properties were correlated to replacement of nitrogen for oxygen in the film bulk. Creation of a nitrogen‐rich layer at the Si/film interface after 10 min of nitridation was correlated to an increase in fixed insulator charge density, . At nitridation times of 30 min and longer, the silicon substrate was reoxidized by water released by the nitridation of the film near the interface. Reoxidation of the silicon steadily lowered .