It is important to understand the fundamental behaviour of reverse osmosis membranes under a range of pH and salinity conditions. In this work, experiments and modelling are used in a complementary manner to better understand these fundamentals. We find experimentally that both pH and salinity can influence membrane charge, the fractional free volume (or pore size) and the membrane thickness. The thickness of the membrane is quantified using ellipsometry while the pore size is estimated through glucose filtration. The membrane swells marginally with increasing pH, but more so with increasing salt concentration up to 170 mM. The flux of hydronium and hydroxide ions are investigated under similar conditions. At zero salinity, the rejection of H3Oþ and OH increases with increasing surface charge density and remains invariant with respect to applied pressure. Upon the addition of salt, negative rejection of both species is observed, corresponding to increasing flux of these highly mobile ions so as to maintain electroneutrality in the permeate solution. In particular, the solute flux of H3Oþ increases significantly with increasing permeate flux, indicative of coupled flow. The Extended Nernst–Planck equation is employed to predict the rejection of these ions, with partitioning at the interface calculated based upon Donnan-equilibrium and sterich indrance. Good agreement with experimental results can be obtained without the adjustment of any parameters.