Positron annihilation spectroscopy and cathodoluminescence were employed to study changes in the microstructural and luminescent properties of pure and doped polycrystalline ZnS. All samples show evidence of defect clustering, when sintered at temperatures up to 400 °C, indicated by an increase in the average positron lifetime. The average lifetime of the undoped material is constant (at about 290 ps) from 300 to 700 °C, and then decreases beyond this temperature, due to the out diffusion of vacancy clusters. In comparison, the average lifetime for MnS doped material decreases beyond 600 °C. This decrease in the average lifetime is correlated with an increase in the luminescent intensity of the 585 nm Mn band, indicating that Mn compensation of zinc vacancies contributes to the decrease in the average lifetime. At sintering temperatures above 400 °C, the average lifetime of TbF3 doped material decreases by ∼3 ps / 100 °C, from a peak value of 285 ps, and then at 700 °C drops an additional 15 ps to 260 ps. At 650 °C a change from the distinct Tb spectrum to a broad band emission is observed It is believed that these effects can be linked to the incorporation of Tb3+ ions on Zn2+ lattice sites.