In this context, it is interesting to estimate how the resulting N enhancement depends on the abundance of N nuclei in the SABRE substrate. The experimental data shown in Figure do not give a complete and clear answer to this question: in this figure, only relative values are presented, but not the actual 𝜀 values of the enhancement. Furthermore, high-field experiments are not suitable for this purpose, because the pulse sequences have been optimized for a complex with two N nuclei, whereas at lower N abundance a fraction of the complexes have only one N nucleus (at low N abundance the fraction of the complexes with two N nuclei become negligible). The parameters of the complexes with one or two N nuclei are quite different; consequently, for such complexes optimal may be achieved for different and values. As a 𝜀 𝜔 1 Δ result, direct comparison of values measured at different N 𝜀 abundance becomes problematic. To get around this problem, we have decided to measure at 𝜀 ultralow magnetic fields, where the 1 H and N nuclei become strongly coupled and "spontaneous" polarization transfer without rf-excitation between them becomes efficient. This is the essence of the SABRE-SHEATH method . It is important to note that at ultralow fields the chemical shift difference of the protons bound to the SABRE complex is of no importance, so that symmetry breaking (and hence, singlettriplet conversion) occurs solely due to magnetic nonequivalence. Furthermore, there is no need to analyse the spin dynamics in additional (?) complexes, which strongly affect the para-to-ortho conversion at high field. Although at ultralow fields we are unable to run experiments to elucidate the relative populations of different spin states since we cannot apply any pulses to the protons, we can measure over a wide range of 𝜀 fields. Comparison of the SABRE field dependences measured as a function of the N enrichment (at the same total concentration of N-Py and 14 N-Py) is shown in Figure . The measured signal enhancement indicates that the SABRE polarization efficiency at ultra-low field increases when the abundance of N nuclei is lowered. It is noteworthy that the position of the maximum of the field dependence also depends on the concentration of labelled substrate, which is due to the difference in the parameters of the spin system of the SABRE complex for each specific solution. We attribute these results to conversion between different forms of H 2 , driven by the interaction with N spins. Such interactions not only give rise to spin order transfer to the nitrogen spins, but also to singlet-to-triplet conversion in bound H 2 , by perturbing the spin state of H 2 . As a consequence, at high concentration of 15 N isotopes, pH 2 is converted to thermally polarized H 2 , which can no longer provides any NMR enhancements. When the abundance of N nuclei is low, the source spin order survives for a longer period of time, giving rise to a stronger 15 N signal enhancement. Our observations also explain why N-NMR enhancement factors are so high, of the order of 30,000 for natural isotopic abundance (which is only 0.365%) . The absolute signal intensity, which is given by the product of the concentration (proportional to , the fraction 𝜂 𝑁 of N containing molecules) and the maximal enhancement , is the highest for large . The reason is that at low 𝜀 𝑜𝑝𝑡 𝜂 15𝑁 𝜂 15𝑁 15 N containing molecules very seldom bind to the SABRE complex, resulting in slow polarization build-us and lower signal intensity. Please do not adjust margins Please do not adjust margins Another effect, which supports our conclusions, is the striking dependence of SABRE enhancement levels on the pH 2 bubbling pressure. The results of SABRE experiments performed at bubbling pressures in a range from 5 to 25 bars are presented in ESI (Figure ). The linear growth of SABRE enhancement with increasing bubbling pressure (which leads to an increasing pH 2 concentration in the sample) demonstrates that the excess of fresh pH 2 in the solution attenuates the negative effect of singlet-triplet conversion. However, it is noteworthy, that even at 25 bar with reasonable gas flow rates, the enhancement level does not reach saturation, which possibly indicates the substantial effect of singlet-triplet conversion even at ultra-low field conditions.