The Opalinus Clay is a claystone considered as potential host rock for deep geological disposal of radioactive wastes in Switzerland and Gemany. During the operational phase of the repository, an increase in underground water pH, from 7.5 to a value as high as 13.5 is expected, due to the release of alkaline plumes from concrete degradation, resulting in possible changes in the mineralogy of the host rock. While clay mineral dissolution is known to occur and affect the hydro-mechanical properties of the host rock, the dissolution of other minerals present in claystone, i.e., quartz and calcite, can also take place. This study focuses on identification of such dissolutions by considering NaOH, Ca(OH) 2 and KOH alkaline solutions at different pHs (8.5, 11.5 and 13.5). A low pH = 8.5 was investigated to consider the gradual increase in pH of the underground water resulting from the progressive infiltration of alkaline plumes. Series of Opalinus Clay samples from the lower sandy facies of Mont Terri site were immersed in these solutions for six months and for one year. It was found that quartz dissolution occurred significantly at pHs higher than 11.5, but also at pHs as low as 8.5. The longer the exposure time, the more significant the quartz dissolution. Calcite dissolution was observed after six months at pH higher than 11.5, and after one year at pH higher than 8.5. Thus, the quartz dissolution seemed to be governed by the exposure time, while the calcite dissolution seemed to depend on both exposure time and pH value, due to carbonation phenomenon. While generally attention has been paid to the effect of alkalinity on clay minerals, this paper meticulously studies the dissolution of quartz and calcite initially present in significant proportions in the Opalinus Clay from the lower sandy facies of Mont Terri site by investigating a wide range of pHs. The dissolution of such minerals will undoubtedly affect the hydraulic and mechanical properties of the claystone. Thereby, it is of paramount importance to study this phenomenon in order to ensure the long-term safety of the nuclear waste disposal. Furthermore, by investigating a wide range of pHs, attention is paid to both the early stage of site water pH increase (low pHs) and the late stage (high pHs).