Iron oxide (alpha-Fe2O3) nanoparticles with very small size, high crystallinity, and narrow size distribution were synthesized by infiltration of Fe(NO3)(3)center dot 9H(2)O as an oxide precursor into mesoporous silica (SBA-15 and SBA-16) molds using a wet-impregnation technique. High resolution transmission electron microscopy shows that during the hydrothermal treatment of the precursor at 140 degrees C for 2 days, stable alpha-Fe2O3 nanoparticles inside the silica pores are formed. Subsequent dissolution of the mesoporous silica molds by NaOH resulted in well dispersed nanoparticles with an average diameter of similar to 4 nm. (C) 2011 Elsevier B.V. All rights reserved.