Fluorine-substituted CaTiO3:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH3). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr4+ in the pristine CaTi(O,F)3:Pr phosphor was partially reduced to Pr3+ under NH3 flow leading to an intensity improvement of ca. 450% compared to CaTiO3:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr4+ to Pr3+.