NEW MATERIAL:An alkoxynaphthylazobenzenesulfonic acid or a salt thereof of the formula (wherein R is an 8-20C linear or branched alkyl; and X is H or an atom or group of atoms capable of yielding a monovalent cation).

EXAMPLE: Sodium 4-[(4-dodecyloxy-1-naphthyl)azo]benzenesulfonate.

USE: A treating agent for simultaneously making hydrophobic and coloring a powder of a pigment, etc.; and a dye for coloring wool.

PREPARATION: 4-[(4-Hydroxy-1-naphthyl)azo]benzenesulfonic acid or its salt is treated with a base such as sodium hydride, and then alkylated with an alkyl halide at -10 to 200°C, pref. 50 to 100°C, in an inert solvent, e.g. N,N- dimethylformamide or dimethyl sulfoxide. After the completion of the reaction, the reaction product is extracted to give a free sulfonic acid, which is washed and freed from the solvent to give a dark brown to dark purplish red powder, i.e., a compd. of the formula.