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Thermogravimetry and differential thermal analysis in combination with FTIR, elemental analysis and mass spectrometry were used to investigate the presence of mutual interactions in 1:2 w/w % solid state fixed dose combination of Ionized and Rifampicin and responses compared with those of pure drugs. The TG/DTA studies were carried out in temperature range ambient to 650˚C, Mass spectra were recorded for the mixture and compared with those of the pure drugs. Ionized was found to undergo two stage weight losses where as Rifampicin showed extensive decomposition in both inert and air atmospheres. The drugs in mixture behave completely different and show multiple decomposition patterns on heating. DTA curves revealed the presence of internal oxidation in the Rifampicin affecting its stability, its weight loss pattern and that of the Ionized present in the mixture. Mass spectra of pure drugs and the mixture recorded under various conditions confirm the presence of mutual interactions of drugs in mixture and suggest that in N2 atmosphere a new compound having definite composition is formed in the temperature range 320 to 450˚C as revealed from elemental analysis and mass spectral data.