Hydrogen gas physisorption in two distinct dinuclear Pt(II) metallacycles was investigated with (2)H nuclear magnetic resonance (NMR) spectroscopy. Hydrogen adsorption isotherm data and (2)H NMR spectroscopy indicated that hydrogen storage occurs via condensation within the cavities of one of the macrocycles and at the interstitial sites in the other. In addition, this study further supports the notion that heat of adsorption and pore size play critical roles in hydrogen storage. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.