Abstract: It has been widely reported in the literature the use of nanometric semiconductors in the treatment of organic effluents via heterogeneous photocatalysis. Nanocrystals of ZnO have been studied as an alternative to TiO2 (the most used semiconductor) and it is agreed that these materials have high photoactivity. This work demonstrates the synthesis of ZnO nanoparticles by a precipitation method followed by different thermal treatment, and the study of nanoparticles photocatalytic activity in Methylene Blue dye degradation under UV light. First, in order to obtain the nanoparticles precursor, a solution of Zn(NO3)2 was prepared and destabilized by the addition of NH4OH, yielding in the precursor powder. XRD analysis showed that the precursor was composed by ZnO and Zn(OH)2. In order to improve the precursor properties, the as-obtained powder was processed by calcination, changing the time and temperature of the treatment, and by hydrothermal annealing, varying the reaction pH. The calcination process resulted in more photoactive materials than the precursor, regardless of the temperature used. On the other hand, the hydrothermal treatment produced less photoactive materials when the precursor was treated in neutral or basic medium. The results demonstrate the applicability of the calcination method for the extension of this work and the obtaining of materials of environmental interest.