Disclosed are cyanine dyes that are useful for labelling and detecting biological and other materials. The dyes are of formula (I): in which groups R3 and R4 are attached to the Z1 ring structure and groups R5 and R6 are attached to the Z2 ring structure, and n=1, 2 or 3; Z1 and Z2 independently represent the carbon atoms necessary to complete a one ring, or two-fused ring aromatic system; at least one of groups R1, R2, R3, R4, R5, R6 and R7 is the group -E-F where E is a single bond or a spacer group and F is a target bonding group; one or more of groups R11, R12, R13 and R14 are independently selected from the group —(CH2)k—W, where W is sulphonic acid or phosphonic acid and k is an integer from 1 to 10. The dyes may be used in fluorescence labelling applications, where the presence of one and preferably multiple water solubilizing groups attached to the 3-position of the indolinium ring reduces dye-dye interactions, and hence dye-dye quenching, particularly where multiple dye molecules are attached to components such as nucleic acids, oligonucleotides, proteins and antibodies.