In order to investigate the alteration products of rhyolitic tuffs under alkaline hydrothermal condition, samples of acidic pyroclastic rocks from Darabad area (N-E Tehran) were selected. Rock samples were examined under petrographic microscope, X-Ray powder diffraction, X-Ray Fluorescence and were eventually powdered and mixed with Na⁺ and K⁺ bearing solutions in hydrothermal autoclaves. The effect of changes in temperature, time and concentration of cations in solution, alteration of aforementioned tuffs have been studied. The solutions with 1.25- 7.5 mol/lit cation concentrations from KOH and NaCl salt have been prepared and used in hydrothermal experiments in 100 °C and 150 °C. The results revealed that in alteration of rhyolitic tuffs, in presence of KOH and NaCl solutions, with increasing temperature, concentration of Na⁺ and pH, the quality of quartz decreases and the quality of albite increases (sodic alteration). At 150 °C  and high concentration of Na⁺ and K⁺ (7.5 mol/lit), quartz dissolves completly, albite changes to orthoclase with cationic exchange of Na⁺ by K⁺ (potassic alteration) and forms hydroxy sodalite (zeolitic alteration). XRD studies proposed cubic system with unit cell dimensions of a₀= b₀= c₀= 8.8760 Å for hydroxy sodalite.