Thermal oxidation of silicon in nitrous oxide (N2O) ambient at pressures from 1 to 4 atm has been studied. We show that the oxidation rate is different from the one predicted by the Deal-Grove model for normal oxidation in dry oxygen. In our case, the dependence observed for the oxide thickness as a function of the oxidation time is of the form x = x(0) + (gammat)(beta), where x(0) is a native oxide thickness. For the temperature range between 900 and 1200 degreesC, and 2 atm of pressure, the activation energy for gamma is around 1.18 eV. In addition, the exponential factor beta (at 1000 degreesC) varies as the square root of the N2O pressure. These results indicate that thermal oxidation in N2O behaves in a completely different way than normal oxidation, very likely due to the influence of chemical reactions in the gas phase, to the catalytic influence of the N2O-SiO2 interface, and to the incorporation of nitrogen into the oxide film itself. The results presented here establish the basis for the development of a more complete model for thermal oxidation of silicon in a N2O ambient.