In-situ Ag K-edge XAFS measurements have been carried out for fully Ag+-exchanged zeolite 4A (Ag-4A) during continuous dehydration under vacuum in the temperature range between room temperature (r.t.) and 300 degrees C. The structural parameters (distance r, coordination number N and root mean square displacement sigma) were derived by a three-shell (short and long Ag-O contacts and Ag-Ag contact) by curve-.tting analysis. The systematic dehydration effect on the structural change in the Ag clusters is studied by in-situ XAFS experiments. Ag+ ions are reduced by removal of water molecules due to evacuation for 20 minutes at r.t. The formation of Ag clusters is suggested by full evacuation at r.t. With increasing dehydration temperature, N of Ag around a central Ag increases. For the Ag cluster formed at 300 degrees C the Ag-Ag contact distance is evaluated to be 2.79 +/- 0.02 angstrom and N of Ag around a central Ag is 4.1 +/- 0.3. The cluster formation is supported by XANES spectra which show that the Ag+ ions in the fully hydrated Ag-4A change to Ag atoms in the dehydrated Ag-4A at 300 degrees C under vacuum.