Photoluminescence (PL) measurements carried out between 10 and 140 K on Cu‐diffused InP showed the presence of the Cu band at 1.216 eV. From the temperature variation of PL intensity the activation energy for the quenching was found to be 77.4 meV. The line shape, linewidth, and configuration coordinate diagram of the defect band have been calculated. The vibration energy of the excited state was found to be 14 meV from linewidth analysis. A coupled phonon energy of 38 meV and a Huang–Rhys factor of 2.13 were obtained from line‐shape analysis. The displacement of the excited state minimum from the ground state was found to be 0.079 A˚ which showed that the lattice relaxation of the Cu‐related defect in InP is small. ©1995 American Institute of Physics.