In this work, the sprays generated by a pressure swirl atomizer were examined for different liquid injection pressures and particularly, the effects of the droplet collision phenomena on the axial evolution of the droplet size distribution were carefully studied. Sizes and droplet velocities were measured by means a phase Doppler particle analyzer and number concentrations and liquid volume fluxes for different droplet classes, were evaluated using a full data post-processing based on the droplet transit time inside the probe volume. The collision rates between droplet classes were calculated and some collision outcomes as coalescence and separation were investigated. In all the sprays studied, the highest collision rates were obtained in the region where both high number concentration and elevated relative velocity between droplets were measured. A progressive increase of the droplet mean diameter was produced along the sprays due to the coalescing collisions. As injection pressure increased, higher droplet collision rates and higher inertia collisions were found. The separation with formation of satellite droplets was a more frequent collision outcome. In addition, bigger axial mean free paths were calculated and hence, a more moderated axial increase of the droplet mean diameter was obtained. (C) 2013 Elsevier Ltd. All rights reserved.