Heterogeneous doping of nanocrystalline ceria films, by controlled in-diffusion along grain boundaries, is explored as a means for modifying the space charge potential and the inhomogeneous distribution of defects in the space charge layer known to control the electrical properties of nanocrystalline electroceramics. Nanocrystalline cerium oxide thin films were grown by pulsed laser deposition and modified by a novel doping technique. Thin diffusion sources were deposited and cations such as Ni²⁺ and Gd³⁺ were in-diffused at temperatures of 700–800 °C along columnar grain boundaries normal to the surface; the resulting diffusion profiles were examined by Time-of-Flight SIMS. The properties of these modified films were compared with as-deposited samples, with in-diffusion resulting in decreased electrical conductivity. It is proposed that the variation in conductivity results from a redistribution of charge carriers in the space charge layers due to a change in the space charge potential.