Abstract: Oxa-alkane polyphosphonic acids corresponding to general formula (I) ##STR1## in which R.sup.1 is a C.sub.1 -C.sub.10 alkyl group, an optionally substituted C.sub.6 -C.sub.10 aryl group, a phenyl- or naphthylalkyl group, a group having the general formula ##STR2## or a group having the general formula ##STR3## R.sup.2 represents H or methyl, A represents the structural unit ##STR4## where n is an integer of from 1 to 20 and M represents H or the cation of a base, to the use of the above compounds as complexing agents and as thresholders, and to compositions containing one or more compounds of general formula (I).