Europium added to silver chloride under a nonoxidizing atmosphere enters the lattice substitutionally in the divalent state. A single silver ion vacancy is associated with each europium ion in a nearest cation position at 93 K and leads to an electron resonance which shows orthorhombic symmetry. At room temperature most of the vacancies are dissociated and the predominant spectrum arises from Eu²⁺ in a field of octahedral symmetry. The temperature variation in the width of lines which constitute the outer fine structure components of the spectrum gives an activation energy of 0.22+or-0.05 eV for the vacancy motion. In heavily doped specimens a single line spectrum is observed from aggregated europium. Eu²⁺ is found to sensitize silver chloride in the bulk. At room temperature irradiation with blue light produces a reduction in the number of Eu²⁺ ions and an optical absorption peak at 550 nm due to silver colloid, Irradiation with red light bleaches the colloid band and restores the original number of Eu²⁺ ions.