The effective-medium approach is used to derive expressions for the thermal conductivity of concentrated, solid-in-liquid suspensions containing varying concentrations of a third component that adsorbs on the solid surface and acts as a steric stabilizer. A novel thermal conductivity cell is designed and constructed to measure the thermal conductivity of sterically stabilized, colloidal suspensions of alumina powder in par&n oil. Reasonable agreement is found between the semi-theoretical predictions and experimental data. INTRODUCIION Predicting the thermal conductivity of a microscopically heterogeneous but macroscopically homogeneous two-phase material is a problem of scientific as well as technological interest. Examples of such materials include filled polymers and polymer composites, coal or ceramic slurries, paints, emulsions, foams and packed-bed reactors. This problem is not a new one, and the voluminous literature that exists has been reviewed and summarized on a number of occasions [see, e.g., Meredith and Tobias (1962), Churchill (1986), Tsotsas and Martin (1987) and Dutta and Mashelkar (1987)]. For a stagnant twophase system, one expects that the average or effective thermal conductivity would depend on the thermal conductivities and relative amounts of the individual phases, the shape, size, sge distribution, orientation and geometric arrangement of the dispersed phase and the amount of contact, if any, between the dispersed material. Although one can write formal expressions relating the thermal conductivity to these different variables (Batchelor, 1974), rigorous results are available only for dilute systems. For concentrated -systems, all that one can derive are lower and upper bounds (Batchelor, 1974). The general approach, therefore, has been to replace the real two-phase system by a geometrically simpler, hypothetical system for which the Laplace equation can be solved analytically to yield the required thermal conductivity. According to Dutta and Mashelkar (1987) for solid-liquid as well as liquid-liquid suspensions, such a procedure provides expressions with good predictive capabilities except in the case of highly concen-