The development of controlled polymerization techniques in the last decade has enabled polymer brushes to be grown from inorganic substrates with precision. Less studied are brushes grown from concave geometries of high curvature, such as mesopores, despite their application potential in the separation sciences. The method used here, surface-initiated, activators-regenerated-by-electron-transfer, atom-transfer radical polymerization (SI-ARGET-ATRP), is used to grow a polystyrene brush grown from aluminum oxide pores of 5 nm diameter, to-date the most confined geometry in which ATRP has been conducted. The brush is characterized by TGA, AFM, and FTIR, the latter two methods applied specifically to the external brush. Additionally, permporometry as well as permeability and retention measurements are used to characterize the graft within the mesopores. We show that the brush length is tunable, that the brush length is solvent-dependent, and we also demonstrate the application potential of this hybrid material as an organic solvent nanofiltration membrane. This new class of membranes shows excellent performance: a toluene permeability of 2.0 Lm(-2) h(-1) bar(-1) accompanied by a 90% rejection of diphenylanthracene (MW 330 g mol(-1)).