The permeation of typical acid dyes C.I. Acid Orange 7 (OR) and Acid Red 88 (ROC) and sodium 4-amino-3-hydroxyl-1-naphtalene-sulfonate through a fibroin membrane was studied by examining the apparent interaction between the dyes and the fibroin membrane. The affinity of ROC for the fibroin membrane in a 0.001 M HCI solution was only 0.2 kcal/mol higher than that of OR. For the permeation of the dyes through the fibroin membrane, diluted HCI and NaCl solutions were used. The amounts of OR permeated increased in the order of pure water 0.01M NaCl 0.001M HCl 0.002M HCl solution. The diffusion coefficient of the dyes in the membrane were estimated using the time-lag method. The permeation coefficients were estimated as usually done. The relationship between the diffusion and permeation coefficients was discussed in terms of adsorption mechanism in fibroin dyeing with the acid dyes. The results strongly suggested that acid dyes can permeate fibroin membranes by the adsorption-diffusion mechanism.