Updated heat flow and temperature distribution maps of the Baikal rift zone and the neighbouring southernmost Siberian platform and Transbaikalian fold belt, at depths of 1, 3 and 5 km, provide further evidence for the complex structure of South East Siberia. The heat flow field correlates well with the structural form of basement highs and intervening hypocentres, with the variations in electrical conductivity, and particularly well with the tectonic structure. Oi the tectonic provinces delineated in the region by their mean heat flow, the Baikal rift zone is the most prominent feature which shows up as a series of relatively narrow high-amplitude regional anomalies. These are associated with rift basins (grabens) which, in this case, display a higher heat flow than the marginal uplifts, unlike the Transbaikalian fold belt where the structural highs show higher heat flow than the intermontane depressions. The regional heat flow anomalies in the Baikal rift zone are mainly due to mantle flux fed by hot asthenospheric diapirs ascending through permeable fracture zones in the lithosphere. Outside the rift zones, the greatest proportion of heat comes from the crust. Local thermal highs are produced by upper crustal sources such as fissure intrusions associated with the rifting or they are associated with salt domes located on the craton. Finally, heat flow anomalies are related to the redistribution of heat in the upper lithosphere due to topographic contrasts, faulting, ground-water circulation and erosional or depositional processes.