2-Oxo-2-alkoxy-5-dialkylphosphono-1,2-oxaphospholanes of the formula ##STR1## in which R.sub.1 to R.sub.7 are hydrocarbon radicals can be prepared from .alpha.,.beta.-unsaturated ketones and 2 or more moles of a dialkyl phosphite. Obviously .alpha.-ketophosphonates are intermediates of this reaction, as these compounds react with 1 or more moles of dialkyl phosphite in yielding the oxaphospholanes too. Both reactions are promoted by alkaline catalysts.The new oxaphospholane derivatives are considerably stable against thermal decomposition. They are compatible with polymeric materials and can be used as flame protecting agent for plastics and resins, for example, in polyurethane foams.