A newly discovered large-pore titanosilicate molecular sieve, ETS-10, has found increasingly important commercial use such as a desiccant in chlorofluorocarbon-free air conditioners. Its useful properties are due to unique three-dimensional channel structure and charge distribution in the material. Although the framework structure of Na-ETS-10 was established recently; the effect of cations on the local pore structure remains to be described. This issue is systematically investigated as a function of proton exchange in the present work using Xe-129 NMR spectroscopy. Different from the behaviors of conventional zeolites such as Y and ZSM-5, the "porosity" of ETS-10 is significantly affected by proton exchange : the effective "pore" diameter and Xe uptake are both reduced due to the formation of defects in 12-member ring channels. Xe adsorption isotherms reveal that energetically different adsorption sites coexist in the 12-member ring channels of Na-ETS-10. Proton exchange reduces the difference by proton delocalization.