Platinum was doped into Ag-O to prepare Pt-Ag-O (Pt composition: 1-5 at.%) film, and its thermal decomposition process was studied by measuring the change in transmitted light intensity while heating the film to 600 degreesC. X-ray fluorescence spectroscopy was used to evaluate the oxygen loss up to 260 degrees. When 1 at.% Pt was doped, the AgO to Ag2O decomposition process at 130-160 degreesC became less evident. When Pt composition was further increased, oxygen loss was somewhat suppressed, at least up to 260 degreesC, and a slight degree of Pt-O thermal decomposition became noticeable at approximately 550 degreesC. Platinum doping effectively modified the Ag-O thermal decomposition process, and giving it potential for use as a temperature control for plasmon-related techniques activated by the generation of Ag particles.