The transport phenomena and heterogeneous chemistry involved in the vapor-phase etching of silicon dioxide were analyzed numerically and experimentally in the classic stagnation-flow geometry. The mass-transfer coefficient h(m) was determined numerically by solving the coupled conservation equations for momentum, mass, and energy. The calculations included temperature-dependent transport properties and multicomponent effects. In the absence of homogeneous chemistry, the rate and order of heterogeneous reactions may be determined through knowledge of h(m) and experimental measures of the etch rate. This analysis was used to quantify surface chemistry for the case of silicon dioxide etching by HF vapor. Over a narrow range of substrate temperature, the rate-limiting step for this process switches from transport to reaction control. The apparent reaction order also changes from first order to very high order over the same range.