Jet fuel cleanliness, in terms of dispersed water and dirt, is of paramount importance to ensure aviation safety. In this study, a process image analyzer has been used to determine size distributions of dispersed water droplets and real and standard test dust particulates in jet fuel, both in the laboratory and under representative full-scale operational conditions. The technique is also applied to monitoring water droplet coalescence in a filter-water separator in the presence of a surfactant known to cause coalescer disarming, again under simulated operational conditions. The measured water and dirt count (number) distributions are exclusively log-normal, whereas corresponding volume size distributions show deviations from log-normal behavior as a result of contributions from the relatively small number of larger particles or aggregated droplet clusters, highlighting the importance of volume-based size analyses. Distinguishing between dispersed water droplets and solid particles has been demonstrated quantitatively, using a cosolvent to solubilize the contribution from free water droplets.