High-resolution static adsorption of nitrogen and argon at 77 K was determined on six microporous carbons and two synthetic zeolites. The adsorption behavior at very low relative pressures such as 10(-6) was examined in close detail. An S-shaped feature was observed in the low-pressure region of most of the nitrogen and argon isotherms on microporous carbons. A novel mechanism for the physical processes involved in the filling of micropores designated as monolayer-induced micropore filling is proposed to account for this behavior. Examination of the low-pressure region of these high-resolution adsorption isotherms also revealed that the ultramicropores are filled in the region of 10(-5) relative pressure.