Permanent refractive index gratings were holographically photowritten into rare earth doped glasses using the 488 rn line of a Ar Ion laser. The rare earths Pr3+, Tb3+, and Eu2+ were examined. Experiments were performed to elucidate the mechanism responsible for the photoinduced index change. A model is presented in which the index change is accomplished through photoionization of the rare earth dopant and subsequent trapping of the free carrier. The viability of these materials for use in optical fiber grating devices is discussed.