Monte Carlo simulations using crystal chemical constraints have been used to elucidate the copper diffusion mechanisms in copper-doped zinc sulfide. Relaxation around the ZnS\Cu2-xS interface allows facile copper diffusion, particularly in the presence of substitutional oxygen or sulfur vacancies. Copper diffusion is responsible for the brightness degradation in this electroluminescent material. Reducing sulfur vacancies and substitutional oxygen species are outlined as ways to reduce the luminescence decay.