Sulfate rocks, particularly gypsum rock as its most common near surface form in the earth's crust, are among the evaporitic rocks due to their solubility can cause serious problems if they are present in the foundation and abutments of reservoir dams. To investigate the solubility of gypsum, gypsum blocks were taken from the Gachsaran Formation outcrops at a reservoir dam construction site in Iran. To perform rock dissolution experiments under pressure and water flow conditions, a pneumatic-hydraulic pressure vessel with internal water circulation was designed and manufactured. Dissolution simulation experiments were performed at a constant temperature (25°C) and for a range of pressures, water flow velocities and water acidities, the solution rate constants for the experimental considered conditions. The results confirmed that, firstly, there are strong direct increasing relationships between the pressure and water flow velocity with the solution rate constant, while these relationships with increasing pH are reversed. Secondly, the values obtained from rock dissolution experiments under atmospheric conditions cannot be trusted as design parameters for dam foundation or abutments, because the solution rate constant values at the affected depth below reservoir dams can be several times higher than the surface value.