<p>Mass wasting events, such as landslides and debris avalanches, are common at volcanic edifices and represent a significant hazard. Hydrothermal alteration is widely recognized as a key process that weakens volcanic rock strength and promotes instability, but its impact on the frictional properties of basal shear zones remains elusive. In this study, we investigate the frictional behavior of volcanic rocks from La Soufrière de Guadeloupe (Eastern Caribbean) to better understand the role of hydrothermal alteration in controlling shallow shear zone stability. We conducted shear experiments on layers of powdered rocks exhibiting different intensities of alteration and mineral assemblages under room-dry and water-wet conditions. Velocity-step and slide-hold-slide sequences were performed to assess frictional stability and frictional healing, respectively. Under the tested boundary conditions, our results indicate that hydrothermal alteration and phyllosilicate formation do not systematically contribute to the frictional weakening of volcanic rocks from La Soufrière; instead, alteration type and water presence are critical. Smectite-rich alteration significantly weakens the shear zones, particularly under water-wet conditions, while halloysite/kaolinite-rich alteration shows higher friction, higher healing, and lower stability in both room-dry and water-wet conditions. Additionally, under dry conditions, the frictional strength of all samples remains relatively unaffected, regardless of the alteration type. We conclude that smectite-rich shear zones, when infiltrated by water, are more prone to slip, increasing the likelihood of volcanic flank instability and collapse. Our findings have important implications for volcanic hazard assessment, stressing the need to monitor and map both hydrothermal alteration processes and water infiltration in volcanic systems.</p>