The formation mechanism of zinc oxide (ZnO) hollow hexagonal nanotowers (h-HNTs) synthesized using a simple hydrothermal method was studied. Crystal growth and dissolution mechanism were correlated to the formation of voids and grain boundaries in the nanotowers and are supported by imaging evidence for the first time. Elevated reaction temperature, various ions present in the reaction medium and these voids are responsible for the adsorption–dissolution process and formation of ZnO h-HNTs.