An experiment was conducted to demonstrate the utility of quantitative fracture flow visualization techniques in the study of entrapped fluid phase (air) dissolution into a flowing phase (water) within a horizontal, transparent, analog rough-walled fracture. The fracture aperture field and phase occupancy were measured using light transmission techniques and then combined to calculate bulk water-phase saturation within the fracture as a function of time. Fracture relative permeability as a function of water-phase saturation showed a smooth power law behavior during dissolution. Periodic step pulses of clear water within the dyed water inflow yielded dye concentration fields that demonstrate channeling induced by the entrapped air phase. Clusters of the entrapped air-phase exhibited three types of dissolution behavior: general shrinkage, interfacial recession along cluster appendages, and cluster splitting. Locations for the advance of the wetting phase (water) into a nonwetting entrapped air cluster on its dissolution are not always correlated with either zones of high mass transfer rate (as inferred from gradients in the pulsed dye concentration fields) or with narrow apertures where the wetting phase has been thought to most easily invade. These results suggest that within an individual cluster of the entrapped phase, fluid pressure is at equilibrium and that the path of cluster shrinkage may be controlled primarily by capillary forces resulting from the full three-dimensional curvature that minimizes surface energy of the phase interface. ¿ American Geophysical Union 1995