The number of coupled protons may be reduced by selective chemical replacement of protons by deuterons. An example is shown for [1-13 C, 2,3-d 2 ]-succinic acid in figure . Although this leads to a beneficial simplification of the proton spin system, deuteration introduces problems in the context of ultralowfield NMR, where the spin-spin couplings are of the same order of magnitude as nuclear Larmor frequencies. In this regime, the rapid quadrupolar relaxation of the deuterium nuclei induces rapid decoherence of the entire spin system, resulting in substantial polarisation losses . Furthermore, singlettriplet mixing effects induced by heteronuclear couplings lead to further losses . Conventional heteronuclear decoupling is inapplicable in the ultralow-field regime, since the nuclear Larmor frequencies are not sufficiently distinct. In order to fully leverage parahydrogen-induced polarization in the microtesla regime, polarization transfer strategies are required that not only decouple disruptive deuterium nuclei, but also suppress singlet-triplet mixing effects, in the regime of ultralow magnetic fields.