N-type and p-type skutterudite samples with the composition Co (x) Ni4-x Sb12-y Sn (y) were synthesized with composition range 0 < x < 2 and 3 < y < 5. Samples were pre-processed by solidification into ingots. Skutterudite phase formation was achieved by mechanical alloying the crushed ingots. The milled powders were consolidated to dense pellets by hot pressing. Thermoelectric measurements showed limited high-temperature performance below 400 A degrees C. Skutterudite decomposition above 250 A degrees C was detrimental to Seebeck coefficient. The thermoelectric transport properties can be tuned by varying the Co and Sn level. The lowest lattice thermal conductivity measured was 1.0 W m(-1) K-1 for the Co level of 1.5. The Seebeck coefficient was positive for Co levels > 0.8 and negative otherwise. Seebeck coefficients were low, ranging from -40 to 58 A mu V K-1. The combination of transmission electron microscopy with electron energy loss spectroscopy and powder X-ray diffraction established that Sn can substitute on 2a and 24g sites in the skutterudite structure. Due to the low Seebeck coefficients, the alloys exhibited low figure of merits (ZT) < 0.05.