The effects of the Na₂S₂O₄-NaHCO₃-Na citrate and 2% Na₂CO₃ treatments on the surface area of allophane and imogolite were investigated. Their mechanisms and the meanings of the measured surface area were inferred. Allophane and imogolite samples (<2μm) were separated from weathered volcanic ashes and pumices by treating them with H₂O₂ and by dispersing them in water (pH 4-4.5) after a sonic wave treatment. These clay samples were treated successively with Na₂S₂O₄-NaHCO3-Na citrate and 2% Na₂CO₃, leached with 1M NaCl, dialyzed and freeze-dried.<BR>The EGME (ethylene glycol monoethyl ether) and N2 (nitrogen) specific-surface-areas of the clays treated with Na₂S₂O₄-NaHCO₃-Na citrate and 2% Na₂CO₃ were much smaller than those of the untreated clays. The large difference was also shown in the amount of water retained at high relative humidity (RH 0.93) between treated and untreated clays. On the other hand, the difference was hardly shown in the amount of water retained at middle and low relative humidities (RH 0.43, and 0.20 or 0.15) between the two clays. These results were interpreted in terms of the difference in the properties of aggregates between treated and untreated clays and the difference in the penetration of water, EGME and N₂ molecules into the aggregates.<BR>The measurement of the optical density, viscosity and flocculation value of the dispersed suspensions of both treated and untreated clays indicated that the treated clays formed the larger and less stable domains in suspensions as compared with those of the untreated clays. On drying, these domains form the more compact and larger aggregates. In such aggregates the penetration of EGME and N2 molecules are suppressed. The capillary condensation of water molecules at high relative humidity are similarly suppressed. On the other hand, water molecules easily penetrate into such aggregates and then the amount of water retained at middle and low relative humidities is controlled only by the size of unit particles (constitution units of aggregates).<BR>Based on the above speculation, it was suggested that the EGME and N₂ surface-areas of allophane and imogolite probably correspond to the surface area of the aggregates of unit particles. From the comparison between measured (the amount of water retained on the treated clays at RH 0.20) and calculated (based on the assumption that water molecules are adsorbed by the monolayer on the outer surface of structural units and that the inner holes are filled up with water molecules) values of water retention, it was inferred that the H₂O surface-area corresponds to almost all of the total surface of individual structural units for allophane, but a part of them for imogolite and allophane-imogolite.