Parahydrogen Induced Polarization (PHIP) is a wellestablished tool to enhance weak NMR signals. PHIP makes efficient use of the spin order of parahydrogen (pH 2 , the H 2 molecule in its nuclear singlet spin state) which is converted into observable NMR signals. Since pH 2 is NMR silent (as it does not have a magnetic moment) suitable chemical processes must be harnessed to convert the spin order into an enhanced NMR signal. Such processes are given by catalytic hydrogenation reactions with pH 2 or by reversible interactions of pH 2 with an organometallic complex . In the first method, hydrogenative PHIP, pH 2 is added to a substrate with an unsaturated C-C bond; when the 'nascent' protons in the reaction product stemming from pH 2 are non-equivalent (chemically or magnetically), one can obtain strong NMR signal enhancements. In the second method, termed Signal Amplification By Reversible Exchange (SABRE), pH 2 and a to-be-polarized substrate bind to an Ir-based complex, where spin order transfer gives rise to polarization of the substrate. An advantage of the SABRE method is that the substrate and pH 2 only bind to the complex transiently, i.e., they are not consumed, and dissociation of substrate from the complex results in the formation of hyperpolarized free substrate molecules in solution. Hence, the substrate can be repolarized multiple times by supplying pH 2 to the solution. The SABRE method can be used to enhance NMR signals of protons , and "insensitive" nuclei such as N and 13 C and to polarize various molecules, notably, biomolecules , metabolites and drugs . Both hydrogenative PHIP and SABRE have been successfully applied to hyperpolarize various compounds; however, the optimization of hyperpolarization experiments still remains challenging. Specifically, reaction conditions have to be optimized, as well as polarization transfer efficiency. In this work, we address one more issue, which turns out to be important in both PHIP and SABRE. Specifically, we address the question: "What is the spin order of H 2 in PHIP/SABRE experiments?". At the first glance, this question seems to make no sense, as we always introduce pH 2 , i.e., two protons in the nuclear singlet spin state, into the chemical reaction. However, this obvious answer holds only in the gas phase, where conversion between pH 2 and triplet H 2 (orthohydrogen, oH 2 ) is a very slow process. For the actual PHIP and SABRE processes the answer is not so obvious because pH 2 binds to a PHIP substrate or SABRE catalyst in such a way that the two protons occupy non-equivalent positions. In this situation, their magnetic or chemical equivalence is broken and singlet-triplet conversion in H 2 becomes operative. It is important to note that the conversion is not equivalent to a simple decay of singlet spin order via relaxation, since hyperpolarized oH 2 can be formed in a strongly non-equilibrium state, as has been confirmed by several groups . The aim of this work is to study in detail spin conversion of H 2 and to characterize its actual spin state. Here we perform the study for a SABRE system (using an Ir-based catalyst) and measure the SABRE-derived enhancement of