We report that part of the InP‐anodic oxide interface states consists of bistable defects under two charge state configurations. Deep level transient spectroscopy measurements after positive, negative, or zero bias cooling of the samples from room temperature to 80 K are used. Slow states induced by defects in the anodic oxide layer are also characterized with a concentration in the range 1017–1018cm−3. The effect of the slow states on the bistable behavior is analyzed. This bistable behavior is related to the modification of the Fermi level pinning after some bias stresses.