The Arač (Aracs) ruined church is located Vojvodina on the left side of the River Tisa. It is an unique standing medieval church monument of Serbia. As a part of the restoring and conservation works, among 2009–2011 the samples of stone and man-made materials of this building were taken and analyzed. Using geological methods, the origin, the technology and the weathering processes of these materials were emphasized. Thus, the most probable origin of the stone material was Buda Mts., close to Budapest; the clayey-silty material for the bricks came from the surrounding old Tisa floodplain sediments, the lime for the binding and rough-casting material was obtained from carving by-products of the stone material and the sand of mortar and plaster was transported from the alluvia of the River Mureş. The microscopic study proved, that two or more plastering and painting phases were executed. The weathering of the building elements began after the outbreak of fire what destroyed the wood roof of the church. On the stone elements, after the fire and the frost caused fissuration, by the leaching of carbonate, important loss of material happened (separation of leaf-like crusts, the smoothing of carved ornaments). On the bricks, the pore fillings with hygroscopic salts from the capillary groundwater contribute to the crumbing of them. On the stones and the plasters, the effect of bio-degradation (by root acid caused corrosion and leaching) was found. These observations give information for specialists to select the appropriate materials for the restoration and the conservation of this important historical monument.