Plutonium-238 dioxide is a potentially useful source of energy for biological purposes such as the cardiac pacemaker. Experiments showed that the neutron contribution to the undesirable external radiation from 238PuO2 can be minimized by preparing the compound from oxygen depleted in 18O. Depleted oxygen for these experiments was prepared in a thermal diffusion column in a two-step operation. The 18O content was depleted from the natural abundance of 0.204 to 0.0016 ± 0.0005%. A yield of 30 n/(sec 106) alpha particles was found for the (α, n) reaction on 18O.