Only wool side dyeing in cotton/wool (80/20) blended fabric with C.I. Reactive Red 195 has been carried out under acidic dye bath. It has been found that the drive force of dyeing of wool fiber with C.I. Reactive Red 195 is related with the sodium sulfonate of the dye. The affinity of C.I. Reactive Red 195 containing sodium sulfonate to wool fiber was ascribed to the ionic interaction of the anionic sulfonate of dye with the ammonium cation of wool fiber under acidic condition and the increase in dye affinity due to alkaline treatment after acidic dyeing was due to the covalent bond newly formed via the vinyl sulfonyl or the monochlorotriaznyl group of the dye stuff.