||In spite of the outstanding adsorption properties of microporous silica and hybrid silica xerogels, the selectivity of these materials towards gases and vapours has been studied to a little extent. In this work we studied selected silica and hybrid xerogels of microporous nature to evaluate their adsorption properties for the following molecular separations: carbon dioxide / methane; ethane/ethylene; and hexane isomers. For the carbon dioxide / methane system a microporous silica xerogel synthesized at pH 2.5 showed the most favourable results, adsorbing 1.8 mmol/g of CO2. In the case of the ethane/ethylene system it was possible to obtain materials with reverse selectivity, that is, while silica xerogels adsorbed preferentially ethylene, some hybrid xerogels adsorbed preferentially ethane. Considering the adsorption of hexane isomers (n-hexane, 3-methyl pentane and 2,2-dimethylbutane), both silica and hybrid xerogels, but particularly the latter, were able to differentiate between the adsorption of the mono and the di-branched isomers.