A spray pyrolysis (SP) procedure has been developed to prepare thin films of KTiOPO4 (KTP), and details of the mechanism were explored. The key to the successful application of this procedure was the choice of a titanium precursor capable of coexisting with orthophosphate and nitrate anions in aqueous solution without the presence of chloride anion. The absence of chloride anion permitted a lower pyrolysis temperature. The nearly stoichiometric KTP films were grown on several types of substrates and characterized by several analytical techniques. The thickness of the films could be varied from 0.1 to 7.0 mu m, and the apparent density was calculated to be about 90 to 95% of the single-crystal value. The effects of the deposition parameters on film composition and morphology were studied by energy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM), and the optical quality was determined spectroscopically. As-deposited films were amorphous and annealing at 700 to 800 degrees C in air was necessary to convert them to the polycrystalline state. The effects of annealing films on different substrates were investigated using x-ray diffraction (XRD), SEM, and EDS. The XRD results indicated that annealing caused interdiffusion at the interface between KTP films and either the quartz or silicon substrates. The resistivities of as-deposited films were close to 2.5 x 10(9) Omega cm, while those of the annealed samples were higher than 10(10) Omega cm. Hall voltage on such high resistivity samples could not be measured with our equipment.