Generalized Lorenz-Mie theory for the scattering of arbitrarily shaped beams by spherical particles has been applied to two standard phase Doppler layouts, employing receiving units at 30-degrees and 150-degrees off-axis locations. It is shown that the particle trajectory effects may lead to inaccurate size measurements for the near-forward receiver and may make the near-backward measurements totally misleading when a large particle size range (1 : 40) needs to be covered. Only limited improvements can be achieved by using two phase-shift signals from a single receiving unit for discrimination. The errors associated with the trajectories are also detrimental to the concentration measurements based on the existing criteria. However, an extended optical system employing two identical receiving units, located symmetrically about the plane of the laser beams, provides a robust solution to the trajectory ambiguity. It can be used to measure correctly the particle size and the particle location in the measuring volume. The difficulties associated with estimating the effective size of the measuring volume as a function of the particle diameter (in order to determine the true size distribution and the particle number density) may also be resolved by employing an extended system. Hence, despite a higher cost, this arrangement is attractive, at least for obtaining some benchmark simultaneous measurements of sizes and velocities in two-phase particulate flows.