Transport properties including electrical resistivity, thermopower, and thermal conductivity of polycrystalline Mo _2.5 Ru _0.5 Sb _7-x Te _x compounds for 0≤x≤1 have been investigated in the 2–1000 K temperature range. Additional information on the concentrations and the scattering mechanisms of the charge carriers as well as on the magnetic properties has been obtained through Hall effect and magnetic susceptibility measurements performed in the 5–300 K temperature range. The enhancement in the Te content results in a decrease in the carrier concentration which is at the origin of the simultaneous increase in the electrical resistivity and thermopower. A single parabolic band model with acoustic phonon scattering enables to explain the compositional and temperature dependence of the thermopower while such a simple model fails to adequately describe the electronic thermal conductivity. This characteristic together with the unusual dependence of the thermal conductivity upon alloying might be a direct consequence of strong phonons-dimers interactions displayed by Mo ₃ Sb ₇ . Magnetic susceptibility data demonstrate that the antiferromagnetically-coupled dimers tend to disappear as x is enhanced, lending further support to a crucial role played by these interactions to understand the thermal transport in these materials. Low thermal conductivity coupled with high thermopower values result in a high dimensionless figure of merit ZT∼0.70 at 1000 K in Mo _2.5 Ru _0.5 Sb _6.5 Te _0.5 positioning this material as an interesting candidate for power generation applications.