In 1991, the Commission of the European Communities launched a series of projects specifically aimed at the understanding of chemical hazards. This was called the STEP Program and covered projects on hazard analysis to determine the consequences of releases of flammable pressurized and liquefied gases. In that frame, the present study concerns the experimental determination of the droplet size and velocity distributions within a jet at the exit of a discharge pipe of a liquefied propane storage tank during a sudden blowdown. For this purpose, a Phase Doppler Particle Analyzer (PDPA) was used to measure liquid drop size and velocity. Due to the high drop concentration in the jet, it was necessary to protect the incident laser beams and the light scattered by particles to make the measurements possible. In such conditions, a set of blow-downs was performed by varying the nozzle diameter, the initial pressure inside the storage vessel and the measurement position in the jet, These tests include on- and off-axis measurements. The results are presented, showing the droplet evaporation and the velocity evolution inside the jet. This work provides original data which can be useful for the validation of vapour cloud dispersion codes.