Formation of parahydrogen-induced polarization was investigated via hydrogenation of propene with parahydrogen on rhodium-modified zeolite Y (0.4[Rh]Na-Y) in the gas phase. The PASADENA (Parahydrogen and Synthesis Allow Dramatically Enhanced Nuclear Alignment) protocol was combined with in situ H-1 MAS NMR under flow conditions for observing adsorbed reactants. Antiphase signals of hyperpolarized propane due to the pairwise incorporation of parahydrogen in propene were observed under MAS as well as static conditions. In the case of MAS rotors completely filled with pure zeolite, rapid relaxation of hyperpolarized propane inside the zeolite cages occurs, which was decreased in rotors with empty gas volume. (C) 2012 Elsevier B. V. All rights reserved.