An atomic beam polarized ion source, used heavily since 1989 for producing polarized H±and D±beams for experiments between 25 keV and 20 MeV, have been modified to accept a Lamb–shift, spin–filter polarimeter. In this source, polarized ground‐state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge‐exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear–spin‐polarized beams at Los Alamos, is designed to monitor the polarization of2S1/2metastable H or D atoms emerging from this cesium canal. Metastable2S1/2atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to a magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, its calibration has been checked, and initial impressions gained from its use are presented. ©1996 American Institute of Physics.