The graphite furnace atomic signal of gallium in pure Ar with a pyrolytic graphite (PG) coated tube contained an unresolved double-peak signal. Thermal decomposition of Ga20(g) produced by carbon-catalyzed decomposition of Ga203(s) and an on-surface reduction of Ga203(s) to Ga(g) are proposed for the atomization mechanisms giving the first and second signals, respectively. A small peak indicating the vaporization of Ga203 was observed in an old PG tube after 220 firings. In the presence of an organic matrix, the signal was enhanced due to the occlusion of Ga20(g) in the porous residue of the pyrolyzed matrix. Addition of oxygen produced a peak on the raising part and another peak on the tailing part of the signal observed in pure Ar. The former peak results from a rapid on-surface reduction of the analyte oxide by active sites renewed on the graphite surface, and the latter peak is the same as the small peak in the old PG tube.