We reconsider the problem of the reionization of the universe by means of stellar sources. Using the model developed by Haiman & Loeb (1997), we focus on the effect of changing basic parameters, such as stellar models and spectra, adopted for finding the ionizing photons production rate. We find that the adoption of suitable zero metallicity models instead of even the most metal poor ones, usually adopted by other authors, significantly modifies the epoch at wich the Universe is reionized by the first stellar generations.