In order to assess the mean porosity of diamond membranes we used fluorescent polystyrene beads with different diameters and proceeded to a dead-end filtration on themembrane through a Buchner filtration system. Diamond has been deposited on two commercial fiberglass filterswith different pore sizes of 2.2 and 0.7 μm. The same conditions and growth times were used on each filter, the diamond film thickness is around 140 nmfor the upstream part of the filter. Three different sizes of polystyrene beads have been used namely 1, 0.5 and 0.1 μm. Fluorescence calibration curves were first plotted for each batch of PS beads, with concentration of PS beads in water ranging between 1 • 10-7 and 1 • 10-2 mol/L (see Supporting information). Then, for mean pore size measurement, stock solutions of each size of PS beads were prepared with concentrations set to a value where the solutions remain translucent. Beginning with the smaller diameter of