The adsorption isotherms for C02 on several proteins were measured over the wide range of pressures by the gravimetric method. The apparent amount of isothermal adsorption was a maximum at just above the critical pressure, and remained constant at higher pressures, the amount differing among casein, gelatin, gluten and ovalbumin. A sharp peak was also observed in the adsorption isotherm for SCF on an inorganic adsorbent with a smooth surface, while finite adsorption at higher pressures seems to be characteristic of physical adsorption on porous adsorbents and of chemical adsorption. The adsorption isotherm for soybean proteins differed from those of the other proteins, exhibiting a gradual increase after falling from the maximum value and showing hysteresis over the range of sub- and super-critical pressures. This hysteresis was insignificant after denaturation of the proteins.