The paper presents original method of quantitative and qualitative determination of gaseous products of polymers thermal decomposition. It is used to determine the toxic potential of emitted gases, based on their emission E (g/g) by determining the FED dose or toxicological index WLC50M. The method is based on the use of a fluidized bed reactor equipped with a resistive heating jacket, which allows to conduct decomposition of a tested material at a strictly assumed temperature. To identify the types of gaseous compounds and their emission, infrared spectroscopy with Fourier transformation, FTIR was used. The proposed method of measuring the thermal decomposition products concentrations allows mimic the conditions of the fire at different stages of its development. FTIR spectra are recorded every 5 s, which enables identifying toxic gases as a function of burning time, in a precisely determined temperature. Using the library of reference spectra it is possible to determine both inorganic and organic compounds concentrations present in thermal decomposition products of the tested composites. The use of a fluidized bed reactor together with the FTIR analyser operating on-line allows for a much more accurate determination of the emissions of organic and inorganic gaseous products at the assumed temperature, compared to the methods currently used. Silicone rubber composites filled with polyhedral oligomeric silsesquioxanes (POSS) were used for the tests. The effect of POSS, also in a synergistic system with melamine polyphosphate (MPP) on toxicity of gaseous thermal decomposition products of tested composites at temperatures of 500, 600, 700 and 800 °C was determined.