Aqueous sols having SnO2 nanocrystalline particles less than 2 nm in mean diameter were prepared by ultra-filtration from original acidic and alkaline sols with particle diameters extending over a wide range of 0.5-8 nm. Blue shifts were observed in ultraviolet absorption spectra of both the sols. The relationship between energy band gap and size distribution in the original and ultra-filtrated sols suggests that the blue shifts depend on not only the particle size but also the chemical state of nanoparticle surfaces in the acidic and alkaline sols.