Eu11-xYbxCd6Sb1, Zintl solid solutions have been prepared by tin flux reaction by employing the elements Eu/Yb/Cd/Sb/Sn in the ratio 11 - x(p):x(p):6:12:30 where x(p) is an integer less than 11 representing the preparative amount of Eu (11 - x(p)) and Yb (xp). Efforts to make the Yb compositions for x exceeding resulted in structures other than the Sr11Cd6Sb12 structure type. The crystal structures and compositions were determined by single crystal and powder X-ray diffraction and wavelength-dispersive X-ray analysis measurements. The title solid-solution Zintl compounds crystallize in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr11Cd6Sb12 structure type (Pearson symbol mC58), and the lattice parameters decrease with increasing ytterbium content. Single crystal X-ray diffraction shows that Yb atoms are not randomly distributed in the Eu sites but have a site preference which can be attributed to size effects. The influence of the rare earth (RE) metal sites on thermal and electronic properties of RE11Cd6Sb12 solid solutions has been studied by measuring their thermoelectric properties from 5 to 300 K after consolidation by either spark plasma sintering (SPS) or hot pressing (HP). Electron microprobe analysis reveals that some of the rare earth metal is lost during SPS; as a result pellets formed through SPS have lower electrical resistivity by an order of magnitude due to increased hole-charge carrier concentrations. While the carrier concentration increases, the mobility decreases due to deficiencies in Eu content. Refinement of powder X-ray diffraction shows that Eu loss is mainly from the Eul crystallographic site, which has a unique coordination suggesting that this site plays a key role in the transport properties of RE11Cd6Sb12.