The formation of BaZrO3 from very fine (70-90 nm) ZrO2 powders and coarser (similar to1 mum) BaCO3 powders has been studied in dry and humid air up to 1300degreesC using TGA/DTA, XRD, SEM, TEM, and EDS microanalysis. In the temperature range 900degrees-1100degreesC, barium is rapidly transported at the surface of the ZrO2 particles and reacts, forming BaZrO3. The compound grows as a concentric layer with gradual consumption of the central ZrO2 particle. The overall formation kinetics of BaZrO3 is well described by a diminishing core model, and the most likely rate-determining step is a phase-boundary process at the ZrO2-BaZrO3 moving interface. The size and shape of the final particles is generally determined by the morphology of the starting ZrO2 particles and not by that of the BaCO3. The reaction is faster in humid air than in dry air, and the activation energy decreases from 294 kJ(.)mol(-1) (dry air) to 220 kJ(.)mol(-1) (humid air). When the fraction reacted is >80-90 mol%, the reaction rate rapidly decreases.