An Acid dyes are water soluble anionic dyes have chemically less ammonium salt and more sodium salt of sulphuric, carboxylic or phenol organic acid. They possess affinity for amphoteric fibres while lack affinity of direct dyes for cellulose fibres. While dyeing, ionic bonding with fibre cationic is found responsible for fixation of colored anions in dyed material. Acid dyes are added to dyeing baths to increase the number of protonated amino-groups in fibres. The chemistry of Acid dyes is complex to understand.