Surface (air-fluid) and interfacial (fluid-fluid) tensions of coexisting isotropic and anisotropic aqueous suspensions of microcrystalline cellulose have been investigated as a function of suspension concentration and ionic strength using the pendant drop method. The surface tension of the individual phases remained constant as concentration was varied within the biphasic region, where the anisotropic surface tension value fluctuated around 72 mN m(-1) (value for pure water) and the cellulose chains in the isotropic phase displayed surface activity as surface tension was ca. 67 mN m(-1). Ionic strength influenced surface characteristics as the surface tension of both phases increases systematically with addition of HCl. Such changes in surface activity are attributed to variations. in surface packing density and distribution of the hydrophilic and hydrophobic segments of the cellulose chains. The interfacial tension measurement contrasts such behavior as it remained constant at ca. 10(-2) mN m(-1) as a function of both concentration and ionic strength. This value is 3 times that predicted theoretically for an isotropic-anisotropic interface, but this disagreement is thought to reflect the poor estimation of interfacial thickness and the importance of polydispersity in interfacial tension measurement.