Recent studies on catalytic micro-combustors and LPCVD (Low-pressure Chemical Vapor Deposition) have shown the necessity to include effects of the Knudsen layer on surface reactions. In the present work, a weakly rarefied flow of a binary gas mixture with surface reactions was analyzed by using Hamel's model of the Boltzmann equation and Nocilla's model of molecule-wall interaction. Particular interest was placed on the interaction between the Knudsen layer and surface reactions. The theoretical results show that the Knudsen layer modifies the incident flux of the molecules striking the surface and consequently the surface reaction rate, while the surface reactions in turn modify the flow structure in the Knudsen layer through the non-zero net flux at the surface. A general rate expression for a one-step surface reaction based on the sticking coefficient was obtained and can be readily extended to more complex surface reaction mechanisms. The assumptions underlying the widely-used Motz-Wise correction formula for surface reaction rate are discussed.