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The technique of atomic layer deposition (ALD) is applied to the structural modification and pore tailoring of ceramic membranes. Ceramic microfiltration membranes with an average pore size of 50 nm are used as substrates, on which Al₂O₃ is deposited by ALD with the purpose to tailor their pore size. The thickness of the deposited Al₂O₃ layer increases with the number of ALD cycles, and it is confirmed by scanning electron microscopy that the pore size of the ceramic membrane decreases with increasing ALD cycles until the pores are completely sealed. It is found that an ultrathin selective layer with a gradient porous structure is formed, whose thickness can be tuned by varying the exposure time of percursors. With the increased ALD cycles, the Al₂O₃-deposited membranes have a decreasing water flux, and an increasing retention to bovine serum albumin. The membrane subjected to 600 ALD cycles has a reduced pure water flux from 1698 L (m² h bar)⁻¹ for an undeposited membrane to 118 L (m² h bar)⁻¹, while its BSA retention increases significantly from 2.9% to 97.1%.