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Solid-state reactions of In 2 O 3 /H-mordenite and In 0 /H-mordenite mixtures (Al/In ) 3) were studied using an atmospheric flow-through microreactor, diffuse reflectance Fourier-transform spectroscopy (DRIFTS), and X-ray powder diffractometry (XRD). The indium(III)oxide/H-mordenite mixture was heated in a flow of 2% H 2 /N 2 gas mixture or pure N 2 to 873 and 973 K, respectively. The indium(0)/H-mordenite mixture was heated in a dry and wet N 2 stream to 673-973 K. The reactions were monitored by analyzing the effluent gas, using mass spectroscopy (MS). The protons of H-mordenite were exchanged for In + cations, indicating that In 3+ was reduced and In 0 was oxidized in the exchange processes. In the process of reductive solid-state ion exchange (RSSIE), the indium was reduced by H 2 . In the oxidative solid-state ion exchange (OSSIE) process, the indium was oxidized by H 2 O. Results substantiate that the ion exchange proceeds through a volatile InOH intermediate. Formation of InOH and its rapid transport within the zeolite crystals requires the presence of water vapor. The In + in the zeolite lattice can be oxidized by O 2 or H 2 O to indium oxycations, most probably to InO + , while the obtained oxycations can be reduced in hydrogen back to In + .