An indirectly coupled open-ended coaxial cavity resonator was used to detect the effect of stearic acid surface treatments on the dielectric properties of powders in the frequency range 2.17-3.79 GHz. The Bruggeman symmetric and Looyenga classical mixing rules and a general mixing model were used to determine changes in permittivity and loss tangent of the inclusions. Surfactant processed powders with stearic acid 0.05-1.36 vol.% induced only a small change in inclusion permittivity but caused a notable change in dielectric losses. The dielectric loss tangents of SiO2 and Al2O3 increased from 2 Chi 10(-4) to 4 Chi 10(-4) and 5 Chi 10(-3) to 6.1 Chi 10(-3), respectively. However, a significant decrease in dielectric loss tangent, from 6.2 Chi 10(-3) to 3.8 Chi 10(-3), was noted with surfactant treated ZrO2. The investigated novel characterization method proved to be able to detect even very slight changes in dielectric properties to be utilized in various types of powdery material characterization, e.g., in composite applications and material quality monitoring. (C) 2014 Elsevier B.V. All rights reserved.