Subnanoscale windows (nanowindows) were donated to the walls of single-wall carbon nanohorns (SWNHs) by oxidation in oxygen at different temperatures of 573-693 K. We determined the accessibility of internal nanospaces of the SWNHs through nanowindows to He, Ar, N-2, CH4, SF6, and C-60. An explicit molecular sieving effect of SWNHs due to the nanowindow was shown.