Transport phenomena of a sulfonated monoazo dye, C.I. Acid Red 88, into water-swollen cellulose membranes have been studied at 25-55 degrees C. The results were analyzed on the basis of a parallel transport theory of surface and pore diffusion developed in the direct dye-cellulose system. The effects of the lower attractive force of the dye for the substrate in parallel diffusion were considered. The maximum value of the distribution coefficient (alpha) was one-tenth of that for direct dyes. The values of the surface diffusivity (D-s) and the pore diffusivity (D-p) were constant over a wide range of ct values; however, the difference between D-s and D-p was small, because the dye molecules aggregate as dimers in the liquid phase in the pores but not in the adsorbent phase. Arrhenius plots of the surface diffusivity showed a linear relation, but the plot of the pore diffusivities showed a curved line with a steeper slope at lower temperatures. The value of the ratio of surface diffusion rate to pore diffusion rate beta (=alpha D-s/D-p) for the acid dye was of the same order as that for direct dyes due to smaller D-p; however, it increased at lower temperatures because of enhanced aggregation of the dye in the liquid phase.