The prediction of the behaviour of particles in turbulent gas flows is a real challenge for the optimization of industrial processes. Our experimental study is based on the characterization of a polydispersed particle-laden jet using a phase Doppler anemometer. The behaviour of the dispersed phase is analyzed by particle size-classes. All particle classes become responsive (St~ 1) within the region investigated (x/D < 50). The measurements show a change in the behaviour of the particles when their Stokes number, based on a characteristic time of the large scale turbulent structures in the jet, becomes smaller than one. Moreover, our measurements highlight the great influence of the particle diameter on the particle mean and fluctuating velocities and on the local particle fluxes.