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Scanning electron microscopy and visual examination were used to study the physical changes occurring with temperature when Ni and Cu were used as modifiers for Se. During the pyrolysis stage of the electrothermal atomizer heating cycle, the Ni or Cu matrix was observed to shrink as the metal salt decomposed to the oxide and then the metal. This may have resulted in physical entrapment of Se, which coprecipitated with the excess of modifier salt during the drying stage, and this could contribute significantly to the stabilizing effect of Ni and Cu. Pyrolysis curves for Se in the presence of Ni or Cu show a characteristic dip, or decrease in thermal stability, at a pre-treatment temperature of about 800 "C. However, replacement of the Ar purge gas by H, during the atomizer drying stage eliminated this dip, and this may be due to chemical or physical effects. During the atomization heating stage, the modifiers formed molten droplets, and analyte atomization was governed by its kinetically controlled release from these droplets. This is considered to be the major cause of analyte stabilization during atomization.