The microstructural development of Ti: LiNbO3 optical waveguides, as a function of annealing time and temperature, was studied by x-ray diffraction, scanning and transmission electron microscopy, and Auger electron spectroscopy. The microstructure evolves in three major stages: oxidation, precipitation and abnormal grain growth, and interdiffusion. The deposited Ti film is oxidized at low temperatures through a series of intermediate TiOx phases until complete oxidation to rutile TiO2 occurs at ∼500 °C. At intermediate temperatures, 500-800 °C, epitaxial precipitates of LiNb3O8 are formed at the rutile/LiNbO3 interface. At this stage abnormal grain growth occurs in the rutile film, causing multivariant epitaxy where all of the grains have a single orientation relationship to the substrate. Subsequent interdiffusion between TiO2 and LiNb3O8 produces a solid solution with the rutile structure which, at these temperatures, appears to coexist in equilibrium with the underlying lithium niobate substrate. This rutile solid solution serves as the source of Ti in the final stage of interdiffusion, which occurs only at higher temperatures (≳ 1000 °C), and leads to consumption of the rutile layer by the substrate. Structural models are discussed for epitaxial grain growth and interdiffusion.