Components for catalysts for the polymerization of olefins, consisting of the product obtained by reacting the product of reaction between(A) a magnesium compound selected from the groups(1) compounds having formula XnMg(OR)2-n wherein X is a halogen atom, a hydroxyl group or an alkyl, aryl or cycloalkyl radical containing 1-20 carbon atoms; R is an alkyl, aryl or cycloalkyl radical containing 1-2