The synthesis of ZnO nanorods/nanowires using a hydrothermal process is reported. Zn(NO3)(2) was used as the precursor in the presence of (NH4)(2)CO3 for the formation of ZnO. We show not only that an abundance of ZnO nanorods/nanowires can be obtained, but also that the addition of NaOH reduces the diameters, giving the resulting nanorods/nanowires higher aspect ratios. Moreover, the growth time can be as short as 30 min, which is much less than that previously reported. The growth of ZnO was also investigated at different hydrothermal temperatures to determine the activation energy for the slower lateral [1120] growth that determines the nanorod/nanowire diameter. The activation energy was calculated to be 4836 cal/mol.