Chemistry associated with the selective oxidation of silicon (Si) vs. tungsten (W) by water (H2O) steam in hydrogen (H-2) has been studied. New kinetics data and equations are obtained for SiO2 growth on Si(100) at the atmospheric pressure and 950-1100degreesC, with the H2O fractional pressure between 0 to 0.8 atmospheres. The selectivity window for oxidizing Si but not W is determined to be p(H2) : p(H2O) >3:1. Oxygen content in W films slightly increased after being processed within the selectivity window. When W films were processed out of the selectivity window, both W oxidation and Si migration from the underlying SiO2 layers into the W layers were observed. (C) 2003 The Electrochemical Society.