Weathering of rocks causes strength loss in rock masses, which induces unexpected failures in rock structures. For a sloped area, weathering of rocks has to be considered for a stable slope design. Slake durability test is the most common method used for determining the weathering of rocks. Although tap water has been used as a slaking fluid in the standard test procedure, rocks are often exposed to groundwater with various pH. This study aims to investigate the effects of pH of testing liquid for slake durability testing and relations between the mineral constituents. To investigate the effect of pH of the test liquid, nine rock samples were subjected to slake durability tests using testing liquid with different pH levels. Additionally, chemical and X-ray diffraction analyses were performed to determine the mineral composition of test samples. The test results show that the mineralogical composition has a great influence on the slake durability of rocks in both acidic and alkali environments.