This paper demonstrates the results of a long-term study on the effect of water saturation on the mechanical properties and failure modes of tuffs and andesites. As an initial step, an extensive literature survey has been performed about the water effect on mechanical strength of rocks. Afterwards, experimental studies have been initiated and five different igneous rock samples (three tuffs and two andesites) were subjected to uniaxial compressive strength, indirect tensile strength, ultrasonic pulse velocity tests under three extreme conditions which are fully saturated, oven-dry and air-dry. Additionally, mass gain or loss of representative samples was tracked for 1-year period and the wetting and drying profiles of the rocks were demonstrated. Effect of the water saturation and natural moisture on the compressive strength, tensile strength, ultrasonic pulse velocity and failure modes of the samples was investigated. Consequently, it has been seen that while compressive and tensile strength of the samples were reduced with the presence of water and natural moisture, ultrasonic pulse velocities have been increased with water saturation. Additionally, andesite samples demonstrated a shear localization during the uniaxial compression due to saturation.