[en] We measured the 0 deg. polarization transfer of the ²H(p→, n→)pp reaction used to produce a high-intensity, monoenergetic neutron beam. The 0⁺ → 0⁺ reaction ¹⁴C(p→, n→) ¹⁴N(2.31) has been used to calibrate a high-efficiency polarimeter, which in turn was used to measure the polarization transfer in ²H(p→, n→)pp. With this data the uncertainty on the polarization of the 70 MeV neutron beam, used for many different studies at PSI, is reduced to 1.1%. The data are compared to predictions of Faddeev calculations based on realistic nucleon-nucleon interactions