We examined the textures, volatile contents, and cooling histories of microlite-defined flow bands in several rhyolitic obsidians in order to test whether textural variations between bands could be ascribed to different degassing and cooling histories, and to assess the timing and location of band formation. Flow bands are defined by variations in microlite number density ( N V ) and size. For each mineral phase examined, smaller average crystal sizes characterize the microlite-rich bands in contrast to microlite-poor bands, which contain relatively larger crystals of lower N V . Magmatic H 2 O concentrations of microlite-rich and microlite-poor bands show no statistical difference between the textures. Calorimetric measurements yield similar glass transition temperatures and cooling rates for adjacent bands. These observations suggest that microlite heterogeneities could not have developed during late stage cooling and degassing during flow emplacement, as such textural variations imply distinct cooling and/or degassing histories. Rather, textural heterogeneities must have formed during flow in the conduit. Hydrothermal annealing experiments were conducted on natural fragmented rhyolite in order to simulate the welding process in silicate melt and to provide first-order estimates of the time scales and deformation required to produce flow bands. Flow bands formed in experiments conducted at H 2 O-vapor pressures of 50 and 100 MPa, and for temperatures ranging from 800 to 850 °C. In each case, bands formed as a result of redistribution of oxide-rich domains and grain boundary coatings in annealed glass powders that underwent viscous deformation. Experiments suggest that bands may form on relatively short time scales (∼7 d) and for small bulk strains (∼1). Band formation may be promoted by high melt-H 2 O concentrations, shear stress, and viscous and frictional heating.