Mechanism of HCI formation during pyrolysis of 4 waste fuels (Virginia mallow, corn straw, sunflower husk and refuse derived fuel (RDF) pellets) was studied. Elemental anal. of the fuels showed the highest amt. of chlorine in RDF and corn straw. A detailed anal. of HCI formation was simulated by using a com. software. The CI content in the fuel had the higher impact on the HCI formation. No significant influence of temp. (850-1000 degrees C) and pyrolysis time (2-10 s) on the HCI content in the pyrolysis gas was found.