To study the mechanism of zinc diffusion in GaAs, we diffused zinc from a ZnAs2 source into Si-doped GaAs samples (n almost-equal-to 1.3 X 10(18) cm-3) at different temperatures (from 575-degrees-C up to 700-degrees-C) in sealed evacuated quartz tubes. The samples are characterized by the depth profile of the photoluminescence at different temperatures. The photoluminescence spectra show characteristic emission associated to deep levels of gallium and arsenic vacancies. A detailed analysis of the spectra demonstrates the role played by vacancies in the Zn diffusion process. The spatial correlation between the luminescence spectra and the Zn concentration obtained from secondary ion mass spectroscopy measurements has been demonstrated.