Abstract: Batch reactor experiments were performed at 150°C, 175°C and 200°C to determine the effect of high pH KOH solutions on the mineralogy of the Opalinus shale. In these experiments, the change in solution quench pH at 25°C solution composition, and mineralogy were monitored as a function of time for up to ≈50 days. Runs were performed in 50 ml titanium hydrothermal reactor vessels. Each reactor was charged with 0.5–5.0 grams of the 80–200 mesh size fraction of Opalinus shale, and 25 ml of solution (0.08 and 0.008 m KOH). Under these high pH conditions, the general sequence of reaction products observed is the formation of phillipsite, followed by K-feldspar ± K-rectorite. Phillipsite is a metastable intermediate that eventually transforms to K-feldspar. This sequence of mineral reaction products is very different from that found in the NaOH system.