Processing and Characterization of Porous TiO2 Coatings.-New types of microstructures of porous TiO2 coatings are prepared by spin coating anhydrous Ti-ethoxide solutions onto Si substrates. During deposition, Ti(OEt)4 in the solution layer reacts with atmospheric moisture to form precipitated particles which yield a porous coating composed of a particle cluster network. The development of microstructure and crystal structure depends on the Ti concentration in the sol and the spinning rate. Faster spinning rates of solutions with lower Ti concentration result in the formation of smaller particles and more compact particle clusters. As-deposited coatings are amorphous and crystallize into the anatase phase at 400 . degree.C. Phase transformation to the rutile type starts at 850 . degree.C and is influenced by the microstructure.