Oxidation characteristics of heavily doped silicon were investigated in dry and wet oxygen ambiente over the temperature range 920°–1200°C for oxide thicknesses of 0.10–1.0µ. Silicon uniformly doped with boron or phosphorus was studied, as well as silicon surfaces predeposited with these two elements. Boron concentrations greater than cause an increase in oxidation rates at all temperatures, the greatest effect occurring in dry oxygen. Above 1000°, no such rate increase occurs for phosphorus doped silicon. At 920°C, however, phosphorus concentrations of or more result in a significant increase of oxidation rates. The latter effect is most pronounced in wet oxygen. The results can be explained satisfactorily by considerations which take into account redistribution of impurities during thermal oxidation of silicon. These considerations also enable a tentative prediction of the effect of heavy doping concentrations of other impurities on the oxidation rate.