ABSTRACT: The formation of deposit and corrosion, which was formed depending on the chemicalcomposition of the geothermal water, is one of the most serious problems at the geothermal systems. Inthis study, ion chromotography (IC) and ınductively coupled plasma optical emission spectroscopy(ICP_OES) analysis were carried out for water samples that are taken off Afyonkarahisar GeothermalHeating System. Analysis of the deposits formed in pipeline was carried out by using X-Ray DiffractionTechnique and total hardness of water was analyzed by TS 266 method. The results of XRD analysisshowed that the deposit of CaCO formed in the suburban geothermal pipelines while the deposit ofcontaining Na Ca(OH) , CaCO and MgAlSi formed in the inner city well water pipelines. Pitting anderosion corrosion occurred in the pipelines running well water decalcified with NaCl. Also same caseoccurred in the geothermal pipelines. The increase of amount of dissolved substance in the return waterindicated that St37 steel was exposed to erosion corrosion. The increase of total hardness favored theformation of deposit