Abstract: A method for treatment of process waste obtained from the synthesis of thiophene or its derivatives, which method produces phosphorus pentasulfide of sufficiently high purity for recycling in said synthesis process, as well as a residue material suited for use as a raw material for fertilizers. In the treatment method, the process waste is subjected to vacuum evaporation in a combination heating/evaporation reactor at 500.degree.-700.degree. C. and under a partial vacuum of 0.01-0.2 atm. To improve the yield of the evaporating phosphorus pentasulfide, sulfur is added to the process waste. The evaporating phosphorus pentasulfide is condensed, and when desired, pulverized. The usable, phosphorus- and potassium-containing evaporation residue is removed from the reactor and cooled for further use.