Optimization of PHIP transfer to heteronuclei broadens the scope of possible PHIP applications by making them more sensitive and more robust to experimental imperfections. For example, the efficiency of enhancing C signals via PHIP has already been demonstrated in pilot in-vivo MRI studies . In general, there are two options for PHIP transfer, in a high magnetic field , or in ultralow magnetic fields (ULF), where protons become strongly coupled to heteronuclei and coherent spin mixing becomes possible. The major advantage of the transfer in high field is its experimental simplicity, as it does not require any field cycling equipment and is thus feasible on almost any modern NMR or MRI system, and substantial levels of heteronuclear polarization in high field have been reported . However, low fields often prove more efficient for PHIP transfer . This is partly due to lower polarization losses induced by singlettriplet mixing at low magnetic fields when the two protons stemming from pH2 become chemically inequivalent in the reaction intermediates or in the product molecule . Polarization transfer in hydrogenative PHIP generally implies the transfer from protons to C nuclei, in particular carbonyl carbons. Such carbons, being isolated from protons, exhibit long T1 relaxation times, and are found in many biologically relevant molecules. It turns out that to convert PHIP efficiently into C magnetization, the magnetic field must drop below 1 µT, which corresponds to the range of ultra-low magnetic fields (ULF).