In this work, the theoretical algorithm of description of thermochemical processes in graphite tube was tested on determination fluorine using electrothermal molecular absorption spectrometry with aluminum as chemical agent. Theoretical results were calculated using software package HSC 6.1. These results confirm the published experimental data about application of chemical modifier (strontium or barium nitrate) for the possibility of fluorine determination using absorption of molecules AIF. According to calculation the application Ba(NO3)2 or Sr(NO3)2 as chemical modifiers allows retaining fluorine on the stage of pyrolysis up to 900-1000 °C due to forming condensed BaF2c or SrF2c respectively At that the formation of gaseous molecules AlFg in the gas phase of the graphite furnace will occur due to reaction between SrFg/BaFg and Alg. The theoretical results and the offered algorithm can be used for development of methods for fluorine determination in complex matrices by electrothermal molecular absorption spectrometry.