The described protocols were first tested on a model system, i.e., a substrate molecule that demonstrates strong PHIP even when only a small amount of it undergoes hydrogenation (Fig. ). As model system we chose maleic acid, produced by catalytic hydrogenation of acetylenedicarboxylic acid (Sigma-Aldrich, A15207). This system is of particular interest for PHIP studies, as it is the simplest spin system for probing polarization transfer to heteronuclei, comprising only two protons and one C nucleus. It should be noted that maleic acid protons are chemically equivalent, which gives an A2 type spin system when there are no C nuclei in the molecule and an AA'X type spin system when a single C is present. We conducted all experiments with compounds having C isotopes in their natural abundance of 1.1%. This means that when describing PHIP transfer to C we have only a single C nucleus per molecule. Thus, for maleic acid we have two isotopologues, with C being either adjacent to the protons ( 13 C1-maleic acid) or with C in carbonyl position ( 13 C2-maleic acid).