The thermoelectric and structural properties of a double filled skutterudite solid solution (Ca x Ba 1− x ) y Fe 4 Sb 12 were investigated. Using X-ray powder and X-ray micro analyses (EPMA) an immiscibility gap was established with a critical point at x≈0.45 and a critical temperature that depends on the filling level (T C =590±5°C at y=0.8 and T C =610±5°C at y=0.9). The thermoelectric properties were measured for samples prepared in four different states: (i) a single phase solid solution (Ca x Ba 1− x ) y Fe 4 Sb 12 ; (ii) a two phase microcrystalline mixture of Ca y Fe 4 Sb 12 and Ba y Fe 4 Sb 12 ; (iii) a single phase structure obtained after annealing of the latter sample at 600°C for 200h; (iv) a spinodally demixed sample after annealing at 400°C for 672h. The thermoelectric properties of the phase mixture (ii) are compatible with data reported for the microcrystalline end members (Ca y Fe 4 Sb 12 and Ba y Fe 4 Sb 12 ), whilst the single phase (i and iii) and spinodally decomposed (iv) samples show increased thermopower and decreased thermal conductivity, similarly to those observed for nano-structured Ca y Fe 4 Sb 12 and Ba y Fe 4 Sb 12 .