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Recent geophysical investigations revealed the existence of zones in the Earth's crust with anomalous low seismic velocities. The nature of these zones is still under debate, although there are some indications that they might be correlated with changes of porosity and hydraulic permeability as well as with the presence of fluids. To test this hypothesis an experimental study of the influence of fluids on the elastic properties of sandstone as a model rock system for crustal conditions was performed. The investigations were conducted at a hydrostatic pressure of 300 MPa and temperature up to 850°C. They revealed the strong influence of different fluids (neutral, acid, alkaline) on the elastic properties of sandstone as a result of mineral reactions, phase transition and of changes of the rock microstructure. To evaluate the phase changes the computer modeling was performed. Together with investigations of the role of volatiles in geochemical processes including the influence of mineralized fluids on mineral reactions these studies will contribute to the petrophysical and geochemical interpretation of geophysical measurements.