The structural hydroxyl content of the lithospheric mantle is crucial as it is thought to play a key role in influencing several other physical parameters (e.g. melting temperature, rheological properties). Furthermore, since hydrogen is considered as a highly incompatible element, the structural hydroxyl content of mantle xenoliths can be associated with metasomatic processes as well. Thus, it is important to characterize the distribution of structural hydroxyl in the lithosphere. In this study we analyzed 63 petrographically and geochemically well-defined upper mantle xenoliths from the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin) using Fourier-transform infrared (FTIR) spectroscopy. The xenoliths show extremely low average structural hydroxyl contents (~0; 31 and 185 ppm for olivine, orthopyroxene and clinopyroxene, respectively) compared with regionally and worldwide reported values. The studied xenoliths reveal anomalous FTIR spectra characteristics and high clinopyroxene/orthopyroxenes structural hydroxyl ratios (average of ~8). Furthermore, there is no correlation between the structural hydroxyl content and other measured physical or chemical properties of the xenoliths. All these unique FTIR characteristics suggest that the Nógrád-Gömör xenoliths were affected by extensive re-equilibration, which may be linked to both pre- and post-eruptive processes. Extension-related pre-eruptive processes, including re-equilibration during extension induced lower hydrogen activity, are most likely to play the key role. However, a pre-eruption mantle metasomatism caused by an agent with low water activity cannot be excluded. Post-eruption cooling may also be significant, as suggested by the higher structural hydroxyl content in olivines from xenoliths hosted in more rapidly cooled volcanic pyroclastic facies. The novelty of our study includes new aspects on how structural hydroxyl may behave in young extensional basins, as well as providing some diagnostic features to indicate that re-equilibration under lower water activity may have occurred. This also contributes to distinguishing the structural hydroxyl loss linked to the pre-eruptive or the post-eruptive periods