It has been shown that, in ultrathin gate oxidation processes, temperature ramp-up in atmospheric thermal silicon oxidation reactors subjects the wafers to severe temperature nonuniformities. The extent of this nonuniformity has been determined via a method based on the principle that for a given type of silicon substrate, in the absence of nonuniform oxygen partial pressure distributions, the oxidation rate of silicon depends only on substrate temperature. By experimentally determining within-wafer and wafer-to-wafer SiO2 growth rate variabilities, and by using an SiO2 growth rate computer simulation model, substrate temperature has been determined.