Use as a label in an electrochemical assay of a compound of general formula I: in which: Fc is a substituted ferrocenyl moiety having at least one ring substituent selected from sulfur-containing groups, phosphorus-containing groups, iodo, chloro, tri C1-C6 alkyl silyl, C1 to C6 fluoroalkyl groups containing two or more fluorine atoms, heteroaryl, substituted phenyl, and cyano, wherein if present as sole substituent the cyano group is located on the proximal cyclopentadienyl ring; Fc' is a substituted ferrocenyl moiety having at least one ring substituent selected from sulfur-containing groups, phosphorus-containing groups, iodo, chloro, tri C1-C6 alkyl silyl, C1 to C6 fluoroalkyl groups containing two or more fluorine atoms, heteroaryl, substituted phenyl, and cyano, wherein if present as sole substituent the cyano group is located on the proximal cyclopentadienyl ring, and may be the same as or different from Fc; X is a C1 to C6 alkylene chain which is optionally interrupted by-O-, -S-, or -NR 5 -, in which R 5 represents hydrogen or C1 to C6 alkyl; Y is a C1 to C6 alkylene chain which is optionally interrupted by -O-, -S-, or -NR 5 -, in which R 5 represents hydrogen or C1 to C6 alkyl, and may be the same as or different from X; Z is a C1 to C12 alkylene chain which may optionally be substituted and/or may optionally be interrupted by -O-, -S-, cycloalkyl having up to 8 ring members, -CO-, -CONR 1 -, -NR 1 CO- or -NR 1 - in which R 1 represents hydrogen or C1 to C4 alkyl; wherein "substituted phenyl" means a phenyl group having at least one substituent selected from the group consisting of halo, C1 to C4 alkyl, nitro, cyano, C1 to C4 haloalkyl, C1 to C4 alkoxy and sulfur-containing groups; wherein a "sulfur-containing group" means a sulfonyl, sulfinyl or sulfenyl group; wherein a "phosphorus-containing group" means a phosphanyl or phosphinyl group; wherein "alkyl" means a straight- or branched-chain alkyl group, optionally interrupted by a heteroatom selected from O, S and N and/or optionally having one or more substituents; or a cycloalkyl group; wherein "cycloalkyl" means a cycloalkyl group having up to eight ring atoms, optionally including one or more heteroatoms; and R is a hydroxyl group or a protected hydroxyl group or a group containing a hydroxyl group or a protected hydroxyl group; Fc is a substituted ferrocenyl moiety having at least one ring substituent selected from sulfur-containing groups, phosphorus-containing groups, iodo, chloro, tri C1-C6 alkyl silyl, C1 to C6 fluoroalkyl groups containing two or more fluorine atoms, heteroaryl, substituted phenyl, and cyano, wherein if present as sole substituent the cyano group is located on the proximal cyclopentadienyl ring; Fc' is a substituted ferrocenyl moiety having at least one ring substituent selected from sulfur-containing groups, phosphorus-containing groups, iodo, chloro, tri C1-C6 alkyl silyl, C1 to C6 fluoroalkyl groups containing two or more fluorine atoms, heteroaryl, substituted phenyl, and cyano, wherein if present as sole substituent the cyano group is located on the proximal cyclopentadienyl ring, and may be the same as or different from Fc; X is a C1 to C6 alkylene chain which is optionally interrupted by-O-, -S-, or -NR 5 -, in which R 5 represents hydrogen or C1 to C6 alkyl; Y is a C1 to C6 alkylene chain which is optionally interrupted by -O-, -S-, or -NR 5 -, in which R 5 represents hydrogen or C1 to C6 alkyl, and may be the same as or different from X; Z is a C1 to C12 alkylene chain which may optionally be substituted and/or may optionally be interrupted by -O-, -S-, cycloalkyl having up to 8 ring members, -CO-, -CONR 1 -, -NR 1 CO- or -NR 1 - in which R 1 represents hydrogen or C1 to C4 alkyl; wherein "substituted phenyl" means a phenyl group having at least one substituent selected from the group consisting of halo, C1 to C4 alkyl, nitro, cyano, C1 to C4 haloalkyl, C1 to C4 alkoxy and sulfur-containing groups; wherein a "sulfur-containing group" means a sulfonyl, sulfinyl or sulfenyl group; wherein a "phosphorus-containing group" means a phosphanyl or phosphinyl group; wherein "alkyl" means a straight- or branched-chain alkyl group, optionally interrupted by a heteroatom selected from O, S and N and/or optionally having one or more substituents; or a cycloalkyl group; wherein "cycloalkyl" means a cycloalkyl group having up to eight ring atoms, optionally including one or more heteroatoms; and R is a hydroxyl group or a protected hydroxyl group or a group containing a hydroxyl group or a protected hydroxyl group;