Reverse osmosis (RO) filtration is an important separation process for water treatment and many industrial applications. Despite many decades of research and development efforts, solute transport through an RO membrane is still not fully understood. This article provides an insight into transport of small and uncharged solutes in RO membranes, with a particular focus on free-volume hole-size of the membrane active skin layer determined by positron annihilation lifetime spectroscopy (PALS). Free-volume hole-size is one of the most important membrane properties governing solute rejection. In fact, the rejection of boric acid (which is a small and uncharged solute) by RO membranes decreases with increasing free-volume hole-radius. In addition to free-volume hole-size, other properties of the active skin layer may also influence the rejection of uncharged small solutes. Thus, future development of PALS or other analytical techniques to characterise the free-volume hole-shape, hole-size distribution, and free-volume fraction of the active skin layer can provide an unprecedented level of insight into the separation of small and uncharged solutes by RO membranes.