Near- and mid-infrared spectroscopy has been used for the characterization of epoxy networks based on diglycidyl ether of bisphenol-A cured with linear aliphatic diols of different chain length in the presence of magnesium perchlorate. Absorption bands due to epoxide, hydroxyl, and ether groups were investigated- The intensity of the hydroxyl absorption and the formation of branched and unbranched ether structures in dependence on the molar ratio and the diol chain length are discussed. A mechanism for the network formation process with respect to diol chain length and concentration is proposed : networks with low density are rapidly formed if long diols are used for modification, whereas the incorporation of short diols into the network takes a longer time but yields networks with a higher density. Glass-transition temperatures of the networks obtained by dynamic thermomechanical analysis confirm the proposed mechanism.