Oil recovery from oily sludge generated from the petroleum industry has been investigated by solvent extraction using four organic solvents, cyclohexane, n-hexanol, n-butanol and kerosene. Special emphasis was placed on the effect of sludge (solid) concentration (C-s) on the oil recovery efficiency (R-o) and partition coefficient (K-D). The KD was found to vary with C-s. This phenomenon can be described as the "solid concentration effect" or "solid effect" (C-s-effect). The C-s-effect can be explained using the surface component activity (SCA) model that we developed previously. In this model, the activity coefficient of solid surface sites (f(s)) was proposed as a function of C-s rather than unity because of interactions among solid particles. Based on the SCA model, we provide a C-s-dependent function of partition coefficient (termed the SCA-partition coefficient function), in which the intrinsic (or thermodynamic) partition coefficient (K-D(0)) independent of C-s can be simulated from extraction experimental data and can be used to characterize the extraction equilibrium. It was confirmed that the SCA-partition coefficient function could describe the C-s-effect observed in our solvent extraction experiments accurately. In addition, the f(s) of the sludge was independent of the solvent in our tests and decreased with increasing temperature. (C) 2014 Elsevier B.V. All rights reserved.