Strombolian eruptions are caused by the bursting of large gas bubbles through essentially stagnant melt that resides in the uppermost part of the conduit. We investigate the physical properties of this shallow melt via a detailed analysis of vesicularity textures in lapilli ejected during a period of moderate-intensity activity at Stromboli volcano (Italy) in 2002. The lapilli show clear evidence that the erupted material is the product of a late-stage dynamic mingling of melts that are distinct in terms of density and rheology. Vesicle-volume distributions for two end-member melts indicate contrasting degrees of outgassing that can be linked to different residence times in the shallow conduit. We propose a model in which actively vesiculating melt rises with the gas phase and mingles with more mature stagnant melt en route to the magma's free surface. We suggest that the complex rheology of this mingled melt feeds back to strongly influence eruption dynamics.