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Half Heusler materials are promising thermoelectric materials with potential application in generators at medium range temperatures. Solid solutions are typically prepared by arc melting, presenting interesting properties. In this work, the effect of Hf incorporation and the formation of solid solutions is discussed. More specifically, Ti1−xHfxNiSn and (Ti0.4Zr0.6)1−yHfyNiSn half Heusler materials were synthesized via mechanical alloying and consolidated via hot press sintering. Hf incorportation in the lattice strongly affected the lattice thermal conductivity due to the large mass fluctuation. The power factor and thermoelectric figure of merit was optimized via Sb doping the values of 34 μW/cmK2 and 38 μW/cmK2; 0.72 and 0.76 at 762 K for Ti0.4Hf0.6NiSn0.985Sb0.015 and (Ti0.4Zr0.6 )0.7Hf0.3NiSn0.98Sb0.02, respectively, were reached.