An ink composition comprising: an oxygen sensitive dye capable of fluorescing in a manner proportional to oxygen content; and a polymer matrix for the dye, characterised in that the polymer matrix comprises units having pendant sulphonic acid or phosphonic acid groups or salts or esters of such groups; wherein the polymer matrix comprises: (a) a copolymer of an acrylic monomer or oligomer and an ethylenically unsaturated compound containing one or more sulphonic or phosphonic groups; or (b) a reaction product of a polymer containing pendant reactive groups with a sulphonic-group-containing compound or phosphonic-group-containing compound of formula or (VIII): HO-X 1 -R 2 -X 2 -Z or H 2 N-X 1 -R 2 -X 2 -Z (VIII) in which: R 1 represents a hydrogen atom or a C 1 - C 6 alkyl group; X 1 represents an oxygen atom, a group of formula -NH- or an alkylene group; R 2 represents an alkylene group, an arylene group, a group -A-B- (where one of A and B is an alkylene group and the other of A and B is an arylene group, an oxyalkylene group or an alkyleneoxy group); X 2 represents an oxygen atom or an alkylene group; and Z represents a group of formula or: -SO 3 H or in which R 3 represents a hydroxy group, an alkyl group, an arylalkyl group, an alkoxy group or a group of formula: in which R 1, X 1, R 2 and X 2 are as defined above; or is a salt or ester thereof.. an oxygen sensitive dye capable of fluorescing in a manner proportional to oxygen content; and a polymer matrix for the dye, characterised in that the polymer matrix comprises units having pendant sulphonic acid or phosphonic acid groups or salts or esters of such groups; wherein the polymer matrix comprises: (a) a copolymer of an acrylic monomer or oligomer and an ethylenically unsaturated compound containing one or more sulphonic or phosphonic groups; or (b) a reaction product of a polymer containing pendant reactive groups with a sulphonic-group-containing compound or phosphonic-group-containing compound of formula or (VIII): HO-X 1 -R 2 -X 2 -Z or H 2 N-X 1 -R 2 -X 2 -Z (VIII) in which: R 1 represents a hydrogen atom or a C 1 - C 6 alkyl group; X 1 represents an oxygen atom, a group of formula -NH- or an alkylene group; R 2 represents an alkylene group, an arylene group, a group -A-B- (where one of A and B is an alkylene group and the other of A and B is an arylene group, an oxyalkylene group or an alkyleneoxy group); X 2 represents an oxygen atom or an alkylene group; and Z represents a group of formula or: -SO 3 H or in which R 3 represents a hydroxy group, an alkyl group, an arylalkyl group, an alkoxy group or a group of formula: in which R 1, X 1, R 2 and X 2 are as defined above; or is a salt or ester thereof.. (a) a copolymer of an acrylic monomer or oligomer and an ethylenically unsaturated compound containing one or more sulphonic or phosphonic groups; or (b) a reaction product of a polymer containing pendant reactive groups with a sulphonic-group-containing compound or phosphonic-group-containing compound of formula or (VIII): HO-X 1 -R 2 -X 2 -Z or H 2 N-X 1 -R 2 -X 2 -Z (VIII) R 1 represents a hydrogen atom or a C 1 - C 6 alkyl group; X 1 represents an oxygen atom, a group of formula -NH- or an alkylene group; R 2 represents an alkylene group, an arylene group, a group -A-B- (where one of A and B is an alkylene group and the other of A and B is an arylene group, an oxyalkylene group or an alkyleneoxy group); X 2 represents an oxygen atom or an alkylene group; and Z represents a group of formula or: -SO 3 H or in which R 3 represents a hydroxy group, an alkyl group, an arylalkyl group, an alkoxy group or a group of formula: in which R 1, X 1, R 2 and X 2 are as defined above; or is a salt or ester thereof..