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 K D 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 KD0 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.