Control
of pore window size is the standard approach for tuning gas selectivity in
porous solids. Here, we present the first example where this is translated into
a molecular porous liquid formed from organic cage molecules. Reduction of the
cage window size by chemical synthesis switches the selectivity from
Xe-selective to CH<sub>4</sub>-selective, which is understood using <sup>129</sup>Xe,
<sup>1</sup>H, and pulsed-field gradient NMR spectroscopy.