The chemical stability of back contact materials and metallisations like Sb2Te3 and Ni:V alloys to CdS/CdTe solar cells was investigated. Ternary and quaternary (Cd-Sb-Te-O, Sb-Ni-Te-O) phase diagrams were calculated using thermodynamic data. By applying these diagrams to CdTs/Sb2Te3 or Sb2Te3/Ni-V interfaces used as back contacts in CdS/CdTe solar cells, predictions could be made about chemical stability and possible interface reactions. The phase diagrams and predicted chemical behaviour were proven by some test reactions. Products and reactions were characterised by XRD and DSC. It was found that the CdTe/Sb2Te3 interface is in thermodynamic equilibrium and no reaction occurs. Sb2Te3/Ni and Sb2Te3/V interfaces are not in thermodynamic equilibrium and reaction products like NiTe, NiTe2 or V2Te3 are found in test reactions at low temperatures (200 degrees C) after very short times (1 h), and in the case of V even at room temperature. It is therefore expected that chemical reactions at these interfaces will lead to both degradation of the Sb2Te3 and the Ni-V layer and may influence the efficiency of CdS/CdTe solar cells.