AbstractSimple atomic models are proposed for three metastable centers in InP, which have been called “M centers”. All undergo “large lattice relaxation” and emit or absorb multiple electrons on changing atomic configuration. The one which has been most thoroughly studied is called M and is introduced by MeV electron irradiation. The second, called M:P, is present in as grown device quality InP and is a problem for MISFET devices. The third, called M:Fe, is present in Fe doped InP that has been processed at high T to produce a p+ region. Our model for M consists of a V−InP+In defect complex in one configuration and 2P+In + V+p in the other configuration. Transformation between these two configurations occurs by nearest neighbor hopping of a P atom between vacant sites. The “negative U” character of the M center is a simple consequence of the P atom hop. Our model for the M:P center is a V+p one configuration and a V−InIn−p in the other, the transformation occurs by In atom hopping. For the M:Fe center we tentatively propose Fe−In + V−In in one configuration and Fe−In V+PP+In for the other, the transformation again occurring by P atom hopping. It is also possible that the Fe occupies a bond centered interstitial position, Fe−2bc, in the first configuration and Fe−InV+P in the second, the transformation occurring by Fe atom motion. A distinction between these two possibilities can be made with a pressure dependence measurement. It is noted that the activation energies for these transformations support the Ballistic Model for vacancy migration.