The study considers the hydrothermal alteration of the volcanic rocks in the Koprulu region (western Turkey). Four alteration zones related to hydrothermal solutions have been determined: unaltered/fresh (UZ), altered (AZ), ferrous (FZ) and ferrous and manganese (FMZ). The distribution of the zones is closely related to the fracture/fault pattern. In addition to microscopic examination and X-ray diffraction, the physico-mechanical properties of the samples were established. A positive correlation was found between the mineralogical alteration index, water absorption and apparent porosity. However, there is an inverse relationship between uniaxial compressive strength and mineralogical alteration index (MI). The good correlation (r (2) = 0.822) between MI and strength ratio (Rs) indicates that MI can be a useful tool in determining the alteration degree of volcanic rocks, which has an important effect on their use in the construction industry.