Abstract: The content of gaseous fluoride in welding fume of basic electrode under certain test conditions was measured.The semiquantitative data for the dehydrogenation of CaF₂ in the form of HF were obtained,and correspondingly,it was affirmed that the dehydrogenation of basic electrode in welding was not appeared mainly in the form of HF.The secondary reaction between HF and elements of higher activity in welding metallurgical process was investigated.It was determined with infrared spectrum,that the small amount of gaseous fluoride in the fume consisted of HF and SiF₄,thereby a chemical reaction mechanism of the dehydrogenation of CaF₂ in the form of HF in welding metallurgical process of basic electrode was established.