Although continuous hydrogenation at a field where the LAC condition is fulfilled provides a relatively efficient and simple approach to PHIP transfer, the time averaging and the fact that the initial polarization propagates non-selectively among all states involved in the LACs naturally limits its effectiveness. A significant optimization of the achievable C polarization may be possible by judicious use of the coherent dynamics of the system at LAC revealed by studying the transfer process with a sufficiently high temporal resolution. Fig. .a shows the dependence of the polarization transfer efficiency on the evolution time in magnetic field 300 nT for maleic acid measured according to protocol 2 to study the coherent nature of PHIP transfer. This field corresponds to the maximum of the field dependence for 13 С2-maleic acid (Fig. . a). For 13 C1-maleic acid we obtain not only coherent polarization transfer in ultra-low field, but also polarization transfer due to the heteronuclear dipolar interactions, as evidenced by nuclear Overhauser effects (NOE) during hydrogenation at 50 μT. This leads to an additional positive polarization in 13 C1maleate, which is independent of the time of evolution in ultralow field. After subtracting this incoherent NOE from the experimentally measured polarization, the resulting oscillations of coherent polarization transfer are in good agreement with the calculated data. For 13 C2-maleic acid, we managed to raise the observed C polarization, P( 13 C), from 1.4% in the experiment with fixed field (protocol 1) to 2.4% for coherent transfer with an evolution time of 0.05 s at magnetic field of 300 nT. However, simulations predict an even larger difference in relative efficiencies, 35% for transfer in a fixed magnetic field and 84% for coherent transfer. The experimentally observed difference between the coherent and fixed field approaches is smaller than expected because of latter's robustness with respect to experimental imperfections. In particular, it does not require precise synchronization and allows for offsets from the optimal value of ULF.