PURPOSE: To obtain a dye compd. capable of bonding the dye to a solid support without deteriorating the pH sensitivity of the dye and preventing the leaching of the dye when the support is utilized as the pH sensor of optical fibers.

CONSTITUTION: The dye compd. is a compd. of formula I, II or III (wherein R is a functional group or a combining functional group having ability to form a stable covalent bond to with a solid support; R1 is H or a substituent; Ar is 1-6C alkyl, substd. 4-hydroxyphenyl or 4-hydroxynaphthyl; and X is >C=O or >SO2), e.g. nitrophenolphthalein. The compd. is obtd. by allowing a compd. of formula IV (where R5, is a functional group or a combining functional group having ability to form a stable covalent bond with a solid support) to react with at least 2 mol equiv. phenol, or 1-naphthol.