A bifunctional polyazamacrocyclic chelating agent of the formula (VIIb): wherein: each Q is independently -(CHR 5) p CO 2 R or -(CHR 5) p PO 3 R 6 R 7; Q 1 is hydrogen; each R is independently hydrogen, benzyl or C 1 -C 4 alkyl; R 6 and R 7 are independently H, C 1 -C 6 alkyl or (C 1 -C 2 alkyl) phenyl; each R 5 is independently hydrogen; C 1 -C 4 alkyl or (C 1 -C 2 alkyl) phenyl; X and Y are each independently hydrogen or may be taken with an adjacent X and Y to form an additional carbon-carbon bond; n is 0 or 1; m is an integer from 0 to 10 inclusive; p is 1 or 2; r is 0 or 1; L is a linker/spacer group covalently bonded to, and replaces one hydrogen atom of one of the carbon atoms to which it is joined, said linker/spacer group being represented by the formula: wherein: s is 1; t is an integer of 0 to 20 inclusive; R 1 is H or NO 2, NH 2, isothiocyanato, semicarbazido, thiosemicarbazido, maleimido, bromoacetamido or carboxyl; and Cyc represents a aromatic moiety, or aromatic heterocyclic moiety, each of said moieties optionally substituted with one or more groups which do not interfere with binding to a biological carrier; or a pharmaceutically acceptable salt thereof. each Q is independently -(CHR 5) p CO 2 R or -(CHR 5) p PO 3 R 6 R 7; Q 1 is hydrogen; each R is independently hydrogen, benzyl or C 1 -C 4 alkyl; R 6 and R 7 are independently H, C 1 -C 6 alkyl or (C 1 -C 2 alkyl) phenyl; each R 5 is independently hydrogen; C 1 -C 4 alkyl or (C 1 -C 2 alkyl) phenyl; X and Y are each independently hydrogen or may be taken with an adjacent X and Y to form an additional carbon-carbon bond; n is 0 or 1; m is an integer from 0 to 10 inclusive; p is 1 or 2; r is 0 or 1; L is a linker/spacer group covalently bonded to, and replaces one hydrogen atom of one of the carbon atoms to which it is joined, said linker/spacer group being represented by the formula: wherein: s is 1; t is an integer of 0 to 20 inclusive; R 1 is H or NO 2, NH 2, isothiocyanato, semicarbazido, thiosemicarbazido, maleimido, bromoacetamido or carboxyl; and Cyc represents a aromatic moiety, or aromatic heterocyclic moiety, each of said moieties optionally substituted with one or more groups which do not interfere with binding to a biological carrier; n is 0 or 1; m is an integer from 0 to 10 inclusive; p is 1 or 2; r is 0 or 1; L is a linker/spacer group covalently bonded to, and replaces one hydrogen atom of one of the carbon atoms to which it is joined, said linker/spacer group being represented by the formula: wherein: s is 1; t is an integer of 0 to 20 inclusive; R 1 is H or NO 2, NH 2, isothiocyanato, semicarbazido, thiosemicarbazido, maleimido, bromoacetamido or carboxyl; and Cyc represents a aromatic moiety, or aromatic heterocyclic moiety, each of said moieties optionally substituted with one or more groups which do not interfere with binding to a biological carrier; s is 1; t is an integer of 0 to 20 inclusive; R 1 is H or NO 2, NH 2, isothiocyanato, semicarbazido, thiosemicarbazido, maleimido, bromoacetamido or carboxyl; and Cyc represents a aromatic moiety, or aromatic heterocyclic moiety, each of said moieties optionally substituted with one or more groups which do not interfere with binding to a biological carrier;