The general expression for the magnetic-field-induced quadrupole splitting is derived fordntransition metal ions in a crystal field of octahedral symmetry. The effective magnetic field may arise from either an applied field or a molecular field. Bonding effects are also treated. The temperature dependence of the quadrupole splitting ΔEQinduced by an applied magnetic field is calculated for57Fe high-spin ferrous and low-spin ferric compounds where the ions are in crystal fields of octahedral symmetry.