Deoxyribonucleoside phosphonates, thiophosphonates and selenophosphonates are obtained by condensation of a difunctional phosphonylating reagent of the formulaR--PXY,in which R is an inert non-cytotoxic organic radical, X is chlorine or Y and Y is a secondary amino group, with a deoxyribonucleoside of which the 5-hydroxyl group and any exo-amino group presents in the base radical are protected, and further condensation with a nucleoside of which the 3-hydroxyl group and any exo-amino group present in the base radical are protected, and then oxidation. The thiophosphonates and selenophosphonates and the intermediates of the first condensation stage are new.