This work focuses on the development of model-based estimation and control algorithms for regulating particle velocity and degree of melting in the high velocity oxygen fuel (HVOF) thermal spray process. Initially, an estimation scheme, which is based on gas phase measurements and the particle velocity and temperature equations, is proposed to estimate the volume-based average of particle velocity and melting degree at the point of impact on the substrate. Then, a feedback control system, which directly uses the estimation scheme, is designed to regulate the volume-based average of the particle velocity and degree of melting by manipulating the inlet gas flow rates to the process. The proposed estimation/control structure is applied to a fundamental model of the industrial Diamond Jet hybrid HVOF thermal spray process and its performance and robustness properties are tested through simulations. (C) 2004 Elsevier Ltd. All rights reserved.