This work studied the compatibility pre-formulated drug-excipient mixtures and tablets of cimetidine by TG and DSC coupled to a photovisualization system. The excipients PVPK30, explocel, microcel MC101, aerosil, and magnesium stearate were studied. Curves for the phase transitions of pre-formulated, microcel- and explocel-cimetidine were obtained. The results showed that the excipients, microcel and explocel, did not produce changes in the thermal behavior of cimetidine. This fact was confirmed by the DSC coupled photovisualization system. The rate constants for the thermal decomposition reaction were determined by an isothermal thermogravimetric (TG) method using the classical Arrhenius equations. The results showed that cimetidine tablets presented a stability higher than the cimetidine-excipient mixtures.