The size control of nanowindows on the graphene walls is indispensable to develop innovative adsorption and separation technologies. As single wall carbon nanohorn (SWCNH) consists of graphene wall, the permeation of ions through the nanowindows can be evaluated with adsorption measurement. We regulated the nanowindow size by use of mild oxidation at 473-523 K for 20-70 h. The explicit low pressure adsorption hysteresis was observed in the N-2 adsorption isotherms of thus-oxidized SWCNHs, suggesting the window size of 0.3-0.4 nm. Moreover, the aqueous phase adsorption measurement for Li+, Na+, K+, Rb+ and Cs+ indicates that the nanowindow size is smaller than about 0.37 nm, being close to the estimated size from N-2 adsorption. (C) 2018 Elsevier Inc. All rights reserved.