Abstract The observed mean of 16 reliable heat-flow values in the Black Sea is 0.92 ± 0.23 μcal/cm 2 /sec. The observed heat flow is about 50 percent of the geophysically relevant heat flow because the rapidly accumulating sediment absorbs a significant fraction of the geothermal flux. In addition, the blanketing effect of the thick low-thermal-conductivity sediments in the Black Sea has further reduced the flux through the seafloor by about 15 percent. The mean heat flux out of the Black Sea, corrected for both thermal refraction and sedimentation, is estimated at 2.2 μcal/cm 2 /sec. The value of heat flowing into the base of the crust is about 1.5μcal/cm 2 /sec, indicating abnormally high upper-manHe temperatures. These high temperatures are perhaps caused by the release of heat in exothermic phase changes from light to dense upper-mantle mineral assemblages. The transitional crustal structure observed beneath the Black and southern Caspian Seas may reflect a phase of sedimentation resulting from intensive erosion in response to vertical crustal move-ments caused by reversible, thermally induced density variations in the mantle.