Amorphous microporous mixed oxides of indium and silicon (AMM-InxSi) have been prepared by our acid-catalyzed sol-gel method. The materials are amorphous with a narrow pore-size distribution with pore widths around 0.7 nm, a surface area about 500-800 m(2) g(-1) (BET) and contain homogeneously distributed In-centers in the silicon matrix (XRD, TEM, EXAFS). These materials have been found to be excellent catalysts for the oxidative dimerization of propene with air to 1,5-hexadiene with high selectivity. The best reaction conditions found are gas phase, normal pressure at 550-600 degrees C. Propene conversion has reached 10% with selectivities > 80%.