Nanostructuring of unfilled CoSb3 Skutterudites has been successful in reducing the thermal conductivity of these materials [1]. First measurements on a mixture of the Skutterudite and an inert ceramic with particle sizes in the nanometer range have shown promising results for a further reduced thermal conductivity by decorating the grain boundaries of the thermoelectric material [2]. A strong influence of additional phases like CoSb2 and pure Sb in the samples has been encountered, too. Therefore in the present work a more detailed investigation of the effect of phase purity and grain size by an intential variation of the phase composition is presented. The impact of a variation in the amount of the ceramics addition and of the phase composition on the thermoelectric properties (κ, σ, S) of pure and doped CoSb 3, respectively, are studied and the results are discussed with respect to an enhancement of the thermoelectric figure of merit.