A method for evaluating E(a) (loss), the apparent activation energy of the analyte loss process during the high-temperature thermal pretreatment stage in electro-thermal atomization atomic absorption spectrometry (ETA-AAS) has been developed. The method is based on extracting information from the declining portion of thermal pretreatment curves (in this case presented as absorbance vs pyrolysis time at various fixed temperatures). Five volatile analytes (As, Pb, Sb, Se, and Sn) have been studied in the presence of tungsten chemical modifier (20-mu-g of W in H2O2). The data on E(a) (loss) have been utilized together with those obtained from treating the atomization peaks E(a) and with literature data on known values of bond energies and enthalpies of certain chemical reactions. Possible mechanisms of analyte losses and atomization are discussed.