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Photo-thermo-refractive (PTR) glass is a Na 2 O-K 2 O-ZnO-Al 2 O 3 -SiO 2 silicate optical glass which also contains fluorine, a small amount of bromine, and dopants that yield photo-sensitivity in the UV range. PTR glass undergoes crystallization of NaF nanocrystals after UV-exposure followed by thermal treatment, resulting in permanent refractive index change. In this study, where we explore only the thermally activated transformations in the UV-unexposed glass, we show that bromine decreases the solubility of NaF, i.e., increases the super-saturation of NaF thus increasing the thermodynamic driving force for crystallization. This feature causes a decrease in the maximum volume fraction of crystallized NaF with decreasing bromine content in the parent glass. The evolution of the glass transition temperature, T g , with increasing isothermal treatment time revealed a minimum resulted from interplay between two concurring processes, liquid-liquid phase separation that led to decrease in T g , and Br-controlled NaF crystallization that acted in the opposite direction. In glasses with lower bromine content, fewer and larger crystals appeared and a surface-initiated crystallization was dominant. A surface layer of F-depleted glass imprinted residual macro-stresses, which were not alleviated by annealing.