Transport phenomena of a binary mixture of dyes with a small affinity onto cellulose into a water-swollen cellulose membrane were studied at 55 degrees C. The results were analyzed on the basis of a parallel transport theory of surface and pore diffusion. The diffusion of dye molecules in a binary dye system could be described by the parallel diffusion model. Although existence of the other dye decreased the equilibrium adsorption of both dyes onto cellulose, the surface and pore diffusivities for the model of both dyes were same as those in the single dye system.