This article discusses the transport mechanisms of water and uncharged solute molecules (PEG) through two new nanofiltration membranes : a pure ceramic (gamma-alumina) and a Nafion layer deposited on a mesoporous titanium oxide substrate. It is shown that in both cases the transport mechanisms are always predominantly convective rather than diffusive, even if the difference between the two components is less important with Nafion than with alumina due to specific chemical interactions between the membrane material and the solutes. This approach was also used to characterize the two membranes morphologically, notably the pore radius of similar to 0.6 nm.