After Zn diffusion into Si-doped GaAs (n almost-equal-to 1.5 X 10(18) cm-3), the Zn-diffused samples are annealed under different conditions: (i) in vacuum, (ii) in arsenic vapor, and (iii) with a Si3N4 mask capping the sample surface. The Zn concentration profiles obtained by secondary-ion-mass spectroscopy and the photoluminescence (PL) spectra taken at different depths below the sample surface are studied in detail. After annealing in vacuum, the steep (p+-n) Zn diffusion front advances into the bulk. We observe that the intensity ratio between the Si donor-gallium vacancy complex (Si(Ga)-V(Ga)) related emission band and the band-to-band (e-h) transition is enhanced in the region ahead of the Zn diffusion front. In contrast, Zn atoms diffuse deeper into the bulk of the samples annealed in arsenic vapor with or without capping layer. These samples show the kink-and-tail (p+-p-n) Zn concentration profiles with a decrease in the intensity ratio around the tail region. The analysis of the PL data suggest a supersaturation of gallium vacancies ahead of the diffusion front of the sample annealed in vacuum and an undersaturation of this defect around the tail region of the samples annealed in As vapor. Our results underline the important role of the nonequilibrium of the defect concentration during the postdiffusion annealing, which permits explanation of the anomalous double profile of Zn by the interstitial-substitutional mechanism.