This invention is in the field of gas detection for sulphur hexafluoride (SF6) gas decomposition products, particularly for the detection of thionyl fluoride (SOF2) and sulphur dioxide (SO2). The invention relates to a novel portable handheld instrument that can readily detect SOF2, SF4 and SO2 in SF6 gas filled electrical equipment and in air. An apparatus for detecting SF6 decomposition products comprising: (a) an inlet for receiving SF6 gas containing SOF2 or SF4; (b) A chamber connected to the inlet and containing a catalyst which converts SOF2 or SF4 into SO2; and (c) an SO2 detector connected downstream of the chamber.