Reversible para-hydrogen-induced polarization (PHIP) is a technique to enhance NMR signals in solution, based on the association/dissociation of para-hydrogen and substrate molecules with/from an iridium catalyst. In this work we present an approach to measure NMR relaxation as well as rate constants of para-hydrogen association/dissociation with/from the iridium catalyst. These rates are important for understanding the different PHIP efficiencies observed for different substrate molecules.