Abstract: Magma-water interaction is defined as one of the most important parameters determining the explosivity of volcanic eruptions. However, accurately quantifying the water content in volcanic products formed during syneruptive / post-eruptive processes remains a significant challenge. This study investigated the thermal expansion and decomposition behavior of obsidian sampled from the Nevşehir Acıgöl Maar, which was subjected to heat treatment without exhibiting perlitic texture development, in order to elucidate the underlying mechanisms and characteristics of the process. The physical and chemical changes that the banded obsidian experienced during its expansion were analyzed using various analytical methods. In this context, FT-IR analysis was used to observe modifications in water components. The mass loss in the sample was quantified using the TG-DTA method, while the textural changes occurring during the obsidian expansion experiment were determined through three-dimensional tomographic imaging technique. The results show that thermal decomposition of volcanic glass involves the releaseof volatiles and both primary and secondary water species across a range of temperatures. This study demonstrates experimentally that hydrous rhyolitic obsidian can generate vesicles that expand at varying rates, particularly inregions where water and volatiles are loosely bound within the glass structure.