The positively charged pions produced in proton–proton collisions at a beam momentum of 1640 MeV/c were measured in the forward direction with a high resolution magnetic spectrograph. The missing mass distribution shows the bound state (deuteron) clearly separated from the pn continuum. Despite the very good resolution, there is no evidence for any significant production of the pn system in the spin-singlet state. However, the σ(pp→π⁺pn)/σ (pp→π⁺d) cross section ratio is about twice as large as that predicted from S-wave final-state-interaction theory and it is suggested that this is due to D-state effects in the pn system