Nanocrystalline MgO has been found to be an efficient dehydrohalogenation catalyst. Its reaction with different chlorobutane isomers or 1-bromobutane results in topochemical transformation of the MgO phase to MgCl2 or MgBr2 accompanied by a sharp decrease in the catalyst surface area and increase in its activity. The resulting magnesium chloride and magnesium bromide are very active catalysts for HCl and HBr elimination from halogenated hydrocarbons. The reaction mechanism changes from predominantly E2 elimination on MgO to mostly E1 elimination over MgCl2 and MgBr2, which act as Lewis acids. It was also found that MgBr2 could be completely converted back to MgO by calcination in oxygen at 500degreesC while MgCl2 oxidation did not occur under similar conditions.