This invention relates to dichelants, in particular compounds having two macrocyclic chelant groups linked by a bridge containing an ester or amide bond, especially compounds of formula Vb ##STR1## (wherein each X which may be the same or different is NZ, O or S, at least two Xs being NZ;each Z is a group R.sup.1 or a group CR.sup.1.sub.2 Y, at least one Z, and preferably 2 or 3 Zs, on each macrocyclic ring being a group CR.sup.1.sub.2 Y;each Y is a group CO.sub.2 H, PO.sub.3 H, SO.sub.3 H, CONR.sup.1.sub.2, CON(OR.sup.1)R.sup.1, CNS or CONR.sup.1 NR.sup.1.sub.2, preferably COOH;m is 0 or 1 or 2, preferably 1; each n is 2 or 3, preferably 2; q is 1 or 2, preferably 1;each R.sup.1 which may be the same or different is a hydrogen atom or an alkyl group optionally substituted by one or more hydroxy and/or alkoxy groups;and D is a bridging group having a molecular weight of less than 1000, preferably less than 500, joining two macrocyclic rings via at least one amide or ester bond) and salts and metal chelates thereof.