Bis-oxalic acid amide esters of the formula A5-O-CO-CO-NH-W2-NHCO-CO-O-A5 in which A5 represents an alkyl group containing 7 to 18 carbon atoms, a phenylalkyl group whose alkyl residue contains 1 to 4 carbon atoms, an alkoxyalkyl group containing 1 to 8 carbon atoms and 1 to 3 oxygen bridges, an ally group or a cyclohexyl group, and W2 represents a 1,3- or 1,4-phenylene group, a 4,4''diphenylene group, a 1,4- or 1,5-naphthylene group or a group

IN WHICH X2 is a bridge member -SO2- or a linear or branched alkylene group containing 1 to 4 carbon atoms, and phenylene nuclei of the said groupings may contain one or two substituents Z2, Z2 being an alkyl or alkoxy group containing 1 to 4 carbon atoms or a halogen atom, and m 0, 1 or 2. In connection with these and the preceding formulae it should also be said that the alkyl groups mentioned may be branched so that compounds thus constituted may be particularly valuable in certain cases.