Silk was dyed with several disperse azo dyes until equilibrium was reached at a constant temperature. When the dye uptake was plotted against the concentration of the unadsorbed dye it showed a basically straight line. That is, the adsorption isotherm was mostly a partition type. On the other hand, the adsorption isotherms followed the Freundlich type, in which the dyes would aggregate in the fiber each other. The affinities of adsorption (-Δμ°) of the disperse dyes on silk were determined by means of the slope of the straight line in the partition type, and the straight line part in the lower dye concentration range in the Freundlich type, respectively. The heat of dyeing was estimated by Clausius-Clapeyron's equation. Adsorptions of disperse azo dyes on silk were exothermic reactions and the heats of dyeing were from -7.79 to -10.37kcal/mol. There was a tendency to have a larger affinity for silk as the Inorganicity/Organicity-value (I/O-value) of the disperse dye wasgetting smaller; that is, the dye was more hydrophobic. The dyeing mechanism of disperse azo dyes for silk can be correlated to the I/O-value. In addition, substituent groups in the dye structure should be taken into account. The alkyl group and the nitro group play important roles, while the alcoholic hydroxyl group is not available for adsorption, that is, its group reduces the dye uptake.