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 PS beads, 5 ml of each solution was filtered. The filtration speed has been set around 2 ml/min. After the filtration of the PS beads' stock solution, the concentration of the filtrate has been assessed by spectrofluorometry. The results are expressed in a percentage with respect to the beta ratio, namely the ratio between the stock solution concentrations and the concentration of beads collected in the filtrate. The beta ratios are reported in Table 1. In the case of the 2.2 μm pore size no clear cut-off appears. For the 0.7 μm pore size and the 1 μm beads, the beta ratio is already extremely low with a value of 3‰. One can note that BDD-PM with very low diamond thickness (140 nm) has been used in this experiment. Taking into account that the pore size is directly related to diamond layer thickness, the cut-off can be tuned through the diamond growth time.