The complexity of technical applications involving particle-laden gas flows often impedes a holistic understanding of the interplay of underlying physical aspects. The usual approach is to isolate phenomena and study them individually, forcing researchers to ignore realistic circumstances. An example is the coating formation process Cold Spray (CS), in which high Stokes number particles are accelerated in a supersonic nozzle and free jet. Along the flow, the particle laden gas undergoes extreme changes, e.g. in velocity, temperature and volume fraction, through a series of interconnected flow events for a poly-disperse distribution of particle sizes. A consequence of this complexity is an under-representation of research on the fluid-mechanics in this field. This manuscript aims to build a bridge between empirical testing in CS and more fundamental aspects of the relevant fluid mechanics: Parameters associated with the multi-phase character of the process are not yet well understood. In this sense, the particle mass loading is often ignored, so are particle-particle interactions and the strong variations of local conditions. Previous work has indicated that there is no conclusive understanding of the phase interaction mechanisms in CS beyond high-Mach number drag laws. In order to start filling this gap, the first experiment for the direct observation of the particle behaviour within a CS nozzle is designed. Rectangular de-Laval nozzles are manufactured from quartz glass and particle tracking velocimetry (PTV) is used to obtain particle velocities and positions at the injection zone, throughout the nozzle and in the supersonic jet under varying operating conditions. This study firstly reports the direct measurement of particle injection, acceleration, and dispersion in CS. It suggests physical explanations based on a statistical evaluation of the observed data and forms the basis for more fundamental understanding of CS fluid mechanics. (C) 2018 Elsevier Ltd. All rights reserved.