Electroosmosis is especially effective in removing liquid from a highly compressible difficult-to-filter solid/liquid mixture. The extent of solid/liquid separation depends on the electroosmotic pressure gradient (E-pg) which is a combination of applied electric field, the surface charge density of solid particles and the flow path structure in the mixture. The larger the E-pg value, the higher the solid content of the treated mixture. A theoretical equation of E-pg for a capillary flow is derived from the Navier-Stokes equation, and its extensions to a flow through porous materials are examined by using experimental E-pg values from constant-current electroosmotic dewatering (EOD), electroosmotic permeation (EOP) and electro-forced sedimentation (EFS) tests. It has been clarified that the specific hydrodynamic resistance alpha has the most effect on the E-pg value. The larger the alpha value of the material, the larger its E-pg value. Modifications to the existing theories were suggested to understand the effect of solid/liquid mixture concentration on the separation under electric field more precisely. (C) 2015 Elsevier B.V. All rights reserved.