This work reports part of an experimental work aimed to characterize the mechanisms of poly dispersion in regions of large variations of flow time scales in simple flow configurations. Simultaneous measurements of velocity, size and mass flux obtained with a phase-Doppler system are reported, which allow the characterization of the developping region of a particle laden jet. The jet has an exit Reynolds number equal to 1,5×104 and is loaded with spherical beads with a Gaussian size distribution centered at 50µm and with a standard deviation equal to 15µm, with a mass loading ratio of 65%. Within the region of the flow considered, even smaller particles are unable to follow the air flow, although variations of particle mean and fluctuating velocities are rather large. The velocity fluctuations of the particles are much larger in the axial direction than in the radial direction which is associated with a very small spreading rate. The rate of dispersion is shown to be hindered for small particle sizes, for which the centerline volume flux increases downstream within the first 6 nozzle diameters. It is further suggested that extra dispersion mechanisms, other than those characteristic of the far field region, are dominant in the developing region due to the inability of the small turbulent time scales of the flow in the vicinity of the nozzle to interact with the particles. Moreover, the effect of shear induced forces is consistent with the present observations.