TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The fluoro-substituted contaminants are treated and formed hydrogen fluoride (HF) is recycled to the electrolysis step. The waste gas originates from the start-up phase of the electrolytic cells, where the content of fluorine (F2) is 70-98 vol.%. The waste gas is a charge of F2 comprising impurities and considered unsuitable for the application in the manufacture of electronic items. The waste gas is a charge of F2 which was produced electrolytically and which is identified to be unsuitable for the application in the manufacture of electronic items. The F2 content is greater than or equal to 5 vol.%. The H2 is used to supplement other fuel. The liquefied petroleum gas, natural gas or town gas is applied as other fuel. The oxygen or an oxygen containing gas is supplied to the treatment chamber. The waste gas is treated at a temperature of 800-1500 degrees C. The gas stream leaving the treatment chamber is subjected to a least one post treatment step, where the post treatment step is selected from the steps of passing the gas stream through a water washer, through an alkaline water washer, and contacting the gas stream with an adsorbent or absorbent, and a low temperature treatment. Preferred Components: The waste gas comprises at least one contaminant selected from metal hydrides, non-metal hydrides, alkyls or alkyl esters of elements of the 3rd, 4th or 5th main group of the periodic system of the elements, organometallic compounds, metal amides, halogens, halogen-substituted compounds, hydrocarbons and carbon monoxide. The waste gas comprises at least one contaminant selected from halogenated carbon compounds, silicium hydrides, germanium hydrides, boron hydrides, phosphane, tetraalkyl silicates, silanes, disilanes, polysilanes, trialkyl boranes, trialkyl phosphines, trialkylphosphites, trialkyl phosphates, triethyl borane, tetrakis dialkylamino titanium, carbon monoxide (CO), NH3, BCl3, CF4, C2F6, F2, SF6, Cl2, WF6, NF3, HBr (where the alkyl is saturated or unsaturated 1-5C groups, optionally substituted by at least one halogen atom), and decomposition products of it. EXAMPLE - Fluorine (F2) and hydrogen (H2) were produced by electrolyzing hydrogen fluoride (HF) in the presence of HF adducts of potassium fluoride (KF) as electrolyte salt at a temperature of 80-100 degrees C in a metallic vessel which serves as electrode. Appropriate voltage (8-10 V) was applied, and the electrolysis was started. F2 and H2 were collected in the respective compartments of the apparatus. F2 was delivered to the semiconductor plant, and H2 was delivered to the burner together with liquefied petroleum gas (LPG) fuel. In the semiconductor plant, F2 was applied as chamber cleaning gas to remove deposits comprising carbon-fluoride polymers and silicon. In the burner, the waste gas containing F2, silicon fluoride (SiF4), volatile fluoro carbons and silica (SiO2) was contacted with oxygen and the fuel. The resulting treated waste gas comprises essentially only HF, SiO2, water and carbon dioxide (CO2) from the volatile carbon compounds and from burnt fuel. The treated waste gas was passed through a water scrubber comprising sodium hydroxide (NaOH). HF was reacted to sodium fluoride (NaF), CO2 was absorbed, mostly as sodium bicarbonate (NaHCO3), and SiO2 was removed from the treated waste gas in solid form. The gas stream leaving the washer was passed to the environment.