The optical and electrical properties of grain boundaries inn‐ andp‐type CdTe have been investigated by analysis of single boundaries within bicrystals and of polycrystalline thin films. The grain‐boundary parameters evaluated include the conductivity activation energy, the boundary diffusion potential, the energy distribution of grain‐boundary states, the minority‐carrier recombination velocity, the majority–carrier capture coefficient, and the optical cross section of grain‐boundary states. Passivation experiments indicate that heat treatment in H2or Li provide effective but temporary passivation inp‐type CdTe, and that similar effects are found for heat treatment in air forn‐type material.