Solid-state devices based on thermoelectric materials can directly convert heat into electricity and may be utilized to make use of waste heat, for example from industrial processes and vehicles. We have been investigating the thermoelectric properties of compounds based on copper antimony sulfide, Cu3SbS4 (famatinite). The system shows a dramatic increase in thermoelectric properties when Sn is substituted for Sb in the system Cu3Sb1-xSnxS4-y (0.0  x  1.0). We have collected neutron diffraction data on two mixed metal compositions in this system and found not only does the local structure differ significantly from the average crystallographic model, but also changes with composition. A total neutron scattering study of the end member compositions is proposed to resolve details of local structures that can be used to aid in the analysis of the mixed metal systems.