Abstract: A method for sterilizing a substantially gas impermeable surface which is contaminated with spores comprises the steps of exposing the surface to a humid gaseous environment to enhance the susceptibility of the spores to subsequent chemosterilization, and then exposing the spores to an amount of gaseous chlorine dioxide in an inert carrier gas effective to sterilize the surface and a method for sterilizing an article contaminated with spores which comprises exposing the surface to a gaseous atmosphere comprised of chlorine dioxide gas and water vapor, wherein the amount of water vapor in said atmosphere is adapted to enhance the susceptibility of said spores to the sporicidal action of chlorine dioxide.