Water flow in partially saturated fractures under thermal drive may lead to fast flow along preferential localized pathways and heat pipe conditions. At the potential high-level nuclear waste repository at Yucca Mountain, water flowing in fast pathways may ultimately contact waste packages and transport radionuclides to the accessible environment. Sixteen experiments were conducted to visualize heat-driven liquid flow in fracture models that included (1) assemblies of roughened glass plates, (2) epoxy replicas of rock fractures, and (3) a fractured specimen of Topopah Spring tuff. Continuous rivulet flow was observed for high liquid flow rates, intermittent rivulet flow and drop flow for intermediate flow rates, and film flow for lower flow rates and wide apertures. Heat pipe conditions (vapor-liquid counterflow with phase change) were identified in five of the seven experiments in which spatially resolved thermal monitoring was performed but not when vapor-liquid counterflow was hindered by very narrow apertures and when an inadequate working fluid volume was used. ¿ 1998 American Geophysical Union