Hierarchically structured zinc oxide was prepared from zinc acetylacetonate by a microwave-assisted process. The zinc oxide formed nanoparticles that are packed in substructured spherical agglomerates with a diameter of 0.5 μm. Nitrogen adsorption, x-ray diffraction, and dilatometry were used to investigate the densification. Ion beam method was applied to prepare cross sections and enable microstructural analysis. Three regimes of microstructural evolution were identified on different scales during sintering. In the first regime, nanoparticles changed morphology and densification occurred only in the interiors of the agglomerates. In the second regime, agglomerates became hollow and built necks. Simultaneously, densification set in on the macroscopic scale. A drastic homogenization of the microstructure was observed that marked the beginning of the third regime, where densification and grain growth occurred.