Volcanic aquifers are regarded as important aquifer systems. They have quite variable chemical, structural, and hydrologic properties. In this study, hydrochemical analysis, multivariate statistical and clustering techniques were used to investigate water quality in Mt. Erciyes stratovolcano aquifer system, in central Turkey. Equilibrium analysis and saturation indices were also calculated to understand the evolution of groundwater chemistry. Twenty-eight physicochemical variables were evaluated for 13 sampling stations for 12 months. The results indicate that the groundwater is generally fresh and has moderately hard to hard character. The major ions with respect to their abundance are in the order HCO3 > SO4 > Cl and Ca> Na> Mg> K. The water type of groundwater is Ca-HCO3 and Ca-Mg-HCO3 type based on hydrogeochemical facies. Gibbs diagram revealed that chemical quality of groundwater is governed by water-rock interactions. The results of saturation index of waters indicated that water samples are almost saturated with carbonate minerals (aragonite and calcite) but undersaturation with SO4 minerals. Heavy metals, F and NO3 concentrations in water samples were within the permissible limits according to the Regulation on Waters Intended for Human Consumption. Hierarchical clustering grouped 13 sampling sites into three clusters based on the similarity of groundwater characteristics. Principal component analysis revealed that groundwater quality is controlled by natural processes. As a result, we showed that water - rock interactions, the dissolution of basaltic minerals and residence time are important in the formation of chemical composition of waters in the volcanic aquifer.