We report experiments which show that the development of high-viscosity volatile-depleted skins of liquid around bubbles has a major effect on bubble growth rates and thus on volcanic conduit flow dynamics. Decompression experiments were performed at room temperature in a cylindrical shock-tube using viscous solutions of gum rosin and acetone (GRA). After an initial transient, the propagation of the flow front follows an empirical exponential law covering most of the final expansion. Theories of magma flow dynamics that rely on a constant liquid viscosity conclude that most of the degassing takes place in a diffusion-limited, decelerating bubble growth regime. By contrast, we assume that viscous resistance to bubble growth is controlled by the high viscosity of the liquid skins around bubbles, and find that, in both explosive GRA and magma flows, bubble growth lies mostly in a diffusion-driven but viscosity-limited, accelerating regime. We infer that in explosive events, diffusion-limited degassing is unlikely to occur before fragmentation. ¿ 2001 American Geophysical Union