We measured the 0 o polarization transfer of the 2 H(p, n)pp reaction used to produce a high-intensity, monoenergetic neutron beam. The 0 + -> 0 + reaction 1 4 C(p, n) 1 4 N(2.31) has been used to calibrate a high-efficiency polarimeter, which in turn was used to measure the polarization transfer in 2 H(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.