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This study investigates electroosmotic flow (EOF) with sodium tetraborate buffer in nanoporous anodized alumina membranes. Membranes with pore diameters ranging from 8 to 100 nm have been fabricated with narrow pore size distributions to systematically investigate the effect of pore diameter on the electroosmosis (EO) pumping down to the electric double layer overlap region. EOF was observed in membranes with pore diameters in and below this region, along with evidence of concentration polarization (CP), which resulted in a significant reduction in flux. The initial flux, though, could be fully recovered by temporarily reversing the flow and dislodging the accumulated ion layer from the feed side of the membrane. Stable pumping for up to 2 h was obtained before any flux reduction caused by CP was observed.