We found a strong influence of the composition of the magnetic material on the temperature dependence of the tunneling magneto-Seebeck effect in M gO based tunnel junctions. We use ab initio alloy theory to consider different F exCo1−x alloys for the ferromagnetic layer. Even a small change of the composition leads to strong changes in the magnitude or even in the sign of the tunneling magneto-Seebeck effect. This can explain differences between recent experimental results. In addition, changing the barrier thickness from six to ten monolayers of M gO leads also to a nontrivial change of the temperature dependence. Our results emphasize that the tunneling magnetoSeebeck effect depends very crucially and is very sensitive to material parameters and show that further experimental and theoretical investigations are necessary. The recently theoretically predicted [1] and experimentally confirmed [2, 3] tunneling magneto-Seebeck (TMS) effect in M gO based tunnel junctions belongs to the new field of spin caloritronics [4,5]. In this field the spin-dependent charge transport is combined with energy or heat transport. This means that the spin degree of freedom is exploited in thermoelectrics [6]. Besides the TMS currently investigated effects in the field of spin caloritronics are the spin-Seebeck effect [7,8], the magneto-Seebeck effect in metallic multilayers [9], the thermal spin-transfer torque [10], the spin-dependent Seebeck effect [11], thermally excited spin-currents [12], and the magneto-Peltier cooling [13]. In this letter we investigate the role of the ferromagnetic lead material on the TMS. We show that not only the temperature dependence but even the sign of the TMS effect depends crucially on the material composition.The TMS effect is the change of the Seebeck coefficient with a change of the magnetic orientation of the ferromagnetic leads relative to each other in a tunnel junction. The size of the effect is given by the TMS ratio