Experimental proof is presented for a hitherto undetected solid-state reaction between the solar cell Material Cn(2)ZnSn(S,Se)(4) (CZTS(e)) and the standard metallic back contact, molybdenum. Annealing experiments combined with Raman and transmission electron microscopy studies show that this aggressive reaction causes formation of MoS2 and secondary phases at the CZTSlMo interface during thermal processing. A reaction scheme is presented and discussed in the context of current state-of-the-art-synthesis methods for CZTS(e). It is concluded that alternative back contacts will be important for future improvements in CZTS(e) quality.