The mechanism of a matrix modification for the trace determination of volatile lead and bismuth by an electro-thermal atomic absorption spectrophotometry was studied by a X-ray diffractometry (XRD). For the investigation of structures, the ash products of the elements were produced by using a palladium as a matrix modifier with or without aluminum or nickel as an auxiliary modifier. The same charring conditions as in the analysis of samples were applied together with much concentrated solution of analytical elements and modifiers in a graphite furnace to get a large amount of the product for XRD. The XRD patterns showed PbPd3 for lead and BiPd3 for bismuth. These mean that the reaction procedures through the charring and atomization were changed from $Pb^{2+}$ ${ ightarrow}$ PbO ${ ightarrow}$$Pb^0$ to $Pb^{2+}$ ${ ightarrow}$ PbO ${ ightarrow}PbPd_3$ ${ ightarrow}$ Pb o for lead and from $Bi^{3+}$ ${ ightarrow}$ BiO ${ ightarrow}$ Bi o to $Bi^{3+}$ ${ ightarrow}$ BiO ${ ightarrow}$ $BiPd_3$ ${ ightarrow}$ $Bi^0$ for bismuth by the addition of modifiers. The volatile elements were stabilized by the formation of palladium alloys through a charring process. Charring temperatures were raised about 500 $^{circ}C$ by the alloying and the atomization was also stabilized for the enhancement of sensitivities.