The reaction pathways in the synthesis of Cu3AsS4 have been studied in the DTA in the range 25-870 degreesC with a heating rate of 10 K/min. Educts, intermediates and products were characterised by X-ray diffraction. Educts were the elements and the binary compounds Cu3-xAs, Cu5-deltaAs2, As4S4, As2S3, the two copper(I) sulfides Cu1.91S, Cu2S, and CuS. The 19 examined educt mixtures are divided by their thermal effects during reaction into three groups. Mixtures of the group I have low copper or copper arsenide contents. Educts in group II are mainly copper arsenides and sulfur, and educts of group III enclose all other mixtures, including that from the elements. In all cases the reaction to Cu3AsS4 proceeds stepwise. The various intermediates are mainly the binary compounds CuS and As4S4 at lower temperatures and at higher temperatures the ternary compounds Cu6As4S9, Cu4As2S5 or Cu12+xAs4+yS13. The synthesis is complete at approximately 600 degreesC. Heating above the melting point of Cu3AsS4 at 694 degreesC increases the amount of by-products. In grinding and ageing experiments copper arsenides react with sulfur, As2S3, and CuS to metastable Cu2As. The reaction between Cu3-xAs + S does not proceed by treatment in a planetary ball mill. As4S4 + CuS do not react during mediumised milling, but the formation of Cu3AsS4 occurs after this treatment in one step in the DTA run. The desoxidation of copper powders is a decisive influence factor for the reaction from the elements.