The influence of substituents on the thermal decomposition of monomeric organic compounds was studied. For this purpose the thermal destruction of dozen or so α-amino acids of the diversified chemical constitution, among others compounds containing the second amine group, additional carboxyl group, amino acids containing hydroxyl or sulfhydryl groups and amino acids connected with five-member heterocyclic ring or existing in the form of hydrates or hydrochlorides were investigated. The analyses were performed using a derivatograph in an air atmosphere, sample sizes were from 50 to 200 mg and heating rate from 3 to 15 K min-1. It has been established that the thermal decomposition of studied compounds occurs in three stages. The temperature ranges, in which the analyzed compounds undergo thermal transformations were established. For evaluation of the thermoanalytical results an advanced multivariate data processing method, principal component analysis (PCA), was used. By this method the influence of the specific functional groups on the thermal decomposition of α-amino acids was determined. The stage of decomposition, in which the thermoanalytic data are the best correlated to the chemical constitution of the compound, is the second stage. It has also been recognized, that better discrimination among the analyzed compounds was obtained for the data set of the DTA.