Our work gives a clear evidence that singlet-triplet conversion in bound H 2 plays an important role in SABRE experiments. Due to the spin dynamics, this conversion becomes fast in the Iridium complex. An important feature of this conversion is that it favours one of the three triplet states, here to the central triplet state, producing polarized oH 2 that does not obey a Boltzmann distribution. This can be unequivocally proven by running polarization transfer experiments with an additional pulse applied to the proton channel. Such experiments allow one to estimate the populations of the three spin states of oH 2 experimentally. In the present case, the additional proton pulse giving rise to a strong additional gain in N signal, which is more than 10-fold in some cases. Hence, studying the conversion process is not a matter of pure curiosity, but it is of great practical importance for the performance of the SABRE method.