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Bragg gratings have been written in a nonhydrogenated Corning's SMF28 telecommunication fiber using ultraviolet (UV) pulses at a high power density from an ArF laser. Some of these gratings were uniformly exposed with light at 193 nm. The isothermal accelerated aging method was used to characterize the thermal decays of both pristine and postexposed grating spectral characteristics. UV postprocessing led to an increase in the stability of the Bragg grating reflectivity meanwhile the shifts in the Bragg wavelengths of the postexposed gratings proved to be higher than those of untreated gratings. It is shown that the isothermal decays of the refractive index modulation cannot be used to predict the annealing induced irreversible shifts in the Bragg wavelengths. Our observations are discussed within the Frame of the current theoretical descriptions of the links that may exist between the writing and the thermal stability of Bragg gratings.