A temperature-dependent MAS NMR study of the proton revealed the mobility of acidic protons in Ag-3-PW12O40. Ag3PW12O40 generated two kinds of protons, which were observed at 6.4 and 9.3 ppm, when the silver salt was partially reduced with hydrogen. The former appeared only when hydrogen existed in the gas phase. The dependence of the line shapes of the two peaks on the spinning rate indicated that the protons observed at 6.4 ppm were mobile even at room temperature, while the protons observed at 9.3 ppm were not. When the temperature was raised in the presence of hydrogen, the line width of the two peaks broadened, and the peaks approached each other. They merged into one peak at 373 K. These results indicate that a proton exchange reaction proceeds between the two kinds of protons. Gaseous hydrogen was not directly involved in the exchange reaction. The catalytic activity of the reduced silver salt is discussed with relevance to the mobility of the two types of protons. Temperature dependence of the line width was not observed for the parent acid, H3PW12O40.