The voltammetric behavior of bulky heteropolyanions belonging to the Dawson family of polyoxometalates, [P2VmW18-mO62] n-(m = 0, 3, 6), has been investigated at electrodes coated with vertically-aligned mesoporous silica thin films with pore openings in the 2-3 nm range. The experiments have been carried out in acidic medium (pH 0.3-1), where the silica surface is positively charged and thus likely to interact with the negatively charged redox probes, and at various ionic strengths of the medium. Better permeability of the films was clearly observed at lower pH values, corresponding to higher densities of positive charges onto the silica walls, and of films with a larger mesopore size, favoring the mass transport rates through the nanoporous membrane. Diffusion of such bulky anionic redox probes was dramatically influenced by the ionic strength of the medium, being especially restricted at low electrolyte concentrations where the thickness of the electrical double layer became of the same order of magnitude or larger than the mesopore radius, giving rise to very low voltammetric signals. The overall negative charge on POMs had few effects on their mass transport through the film, except at low ionic strength where the most highly charged redox probes gave rise to the lowest current responses.