Preparation of 2,5-dioxo-1,2-oxa-phospholanes having general formula (I) ##STR1## wherein R.sup.1 represents an alkyl group being optionally substituted and having up to 18 carbon atoms, a cycloalkyl group having up to 8 carbon atoms, an alkenyl group having up to 8 carbon atoms, an aryl group having up to 14 carbon atoms being possibly substituted by lower alkyl groups, by alkoxy groups, by halogen or by lower alkyl radicals, by alkylated or dialkylated amino groups, or which represents an aralkyl group having up to 15 carbon atoms and being possibly substituted in the same way as the aryl group,Wherein R.sup.2 stands for a lower alkyl group or hydrogen and R.sup.3 stands for a lower alkyl radical, a phenyl radical being possibly substituted by halogen or lower alkyl groups, for a benzyl radical or for hydrogen.