Magma degassing is thought to play a major role in magma fractionation, transport, storage, and volcanic eruption dynamics. However, the conditions that determine when and how magma degassing operates prior to and during an erup- tion remain poorly constrained. We performed experiments to explore if the initial presence of gas bubbles in magma influ- ences the capability of gas to escape from the magma. Vesic- ulation of natural H2O-poor (<<1 wt.%) silicic obsidian glasses was investigated by in situ, high temperature (above the glass transition) experiments using synchrotron-based X- ray tomographic microscopy with high spatial (3 μm/pixel) and temporal resolution (1 second per 3D dataset). As a vali- dation, a second set of experiments was performed on identi- cal starting materials using a Karl-Fisher titration setup toquantify the amount of extracted gas that escapes via volatile diffusion and/or bubble coalescence during vesiculation. In both sets of experiments, vesiculation was triggered by heating the samples at room pressure. Our results suggest that the presence of pre-existing gas bubbles during a nucleation event significantly decreases the tendency of bubbles to coa- lesce and inhibits magma outgassing. In contrast, in initially bubble-free samples, the nucleation and growth of bubbles is accompanied by significant coalescence and outgassing. We infer that volatile-undersaturated (i.e. bubble-free) magmas in the reservoirs are more likely to erupt effusively, while the presence of excess gas already at depth (i.e. bubble-bearing systems) increases the likelihood of explosive eruptions.