The high-velocity air-fuel process (HVAF) is an emerging technology used in the thermal spray industry. The Praxair HVAF process combines air and a liquid fuel (e.g., kerosene, diesel) to generate an energy source with extremely high gas velocities. Analytical studies were conducted to investigate gas and particle dynamics in the Praxair HVAF process for coating with WC-l2Co and stainless steel powders. The mass, momentum, and energy conservation equations were first solved, using the TORCH computer program. Typical output from the model includes temperature and velocity profiles as a function of radial and axial position. The PROCESS gas/particle computer program was then used to calculate from these temperature and velocity profiles the dynamics of particles injected into the gas plume. The primary result of the gas/particle code is a description of the injected particle temperature and velocity as a function of position in the plume. A thorough understanding of the process was obtained using this modeling technique. The results of the modeling were confirmed with process diagnostics. Particle temperature measurements for the WC-Co powder system were obtained with a two-color pyrometer; particle velocity measurements were obtained using particle imaging velocimetry. The coatings produced in the study exhibit superior quality, with high-density, high-hardness, low-oxide content, and high-bond strength.