While the high-pressure and high-temperature behavior of natural zeolites has been intensively studied in the last decades by several researchers, to the best of our knowledge, no in-situ X-ray diffraction studies have been performed combining the effects of both. Experiments at these conditions could have crucial geological implications: during the early stages of the diagenetic process (i.e., zeolite facies), both pressure (P) and temperature (T) increase, following different thermal gradient (e.g., about 25°C/Km for the first few kilometers of the continental crust). In this proposal, the PT behavior of two natural zeolites, laumontite ([(Ca4-xNax)Kx][Al8Si16O48]⋅(H2O)n, with n < 16) and offretite (K, Ca, Mg) Si13Al5O36 ·15H2O which occur in a wide range of natural environments, including sedimentary deposits or volcanoclastic sequences interested by burial diagenesis/metamorphism, as well as in hydrothermal vugs of volcanic rocks.