The use of species-specific mass spectral ion signal temperature profiles for generating isoconversion effective activation energies by the Flynn-Wall-Ozawa method is described. Effective activation energies derived from mass-loss and mass-spectral results obtained from TG-MS analyses of calcium oxalate monohydrate and poly(vinyl butyral) thermal decompositions are compared. Although effective activation energies derived from mass-loss data may suggest changes in overall reaction processes, species-specific effective activation energy variations provide information regarding selected reaction mechanisms.