Discrete and chain-like aggregates of well-defined hollow TiO(2) microspheres are reproducibly synthesized in high yield by a modified fluoride-mediated self-transformation strategy using urea as a base catalyst. The shell walls are composed of agglomerated polyhedral anatase nanocrystals, and exhibit hierarchical porosity. The addition of urea tunes the nucleation dynamics and surface states of the elementary TiO(2) building blocks, which together promote the formation of metastable solid microparticles of uniform size that subsequently transform into morphologically invariant hollow microspheres and chain-like aggregates. The high surface area, bimodal mesoporosity of the shell walls, and increased band gap of the hollow TiO(2) microspheres give rise to increases in photocatalytic activity when compared with anatase nanoparticles and aggregates prepared in the absence of urea.