We report measurements of the energy transfer between Er3+ and Ce3+ in Y2O3. The transition between the Er3+ 4I11/2 and 4I13/2 excited states can be stimulated by energy transfer to Ce3+, augmenting the population in the 4I13/2 state at the expense of that in the 4I11/2 state. Experiments were performed on Y2O3 planar waveguides doped with 0.2 at.% erbium and 0-0.42 at.% cerium by ion implantation. From measurements of Er3+ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10-18 cm3/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y2O3 is two times smaller for Er3+ ® Ce3+ than that for Er3+ ® Eu3+ in the same material.