Twenty new heat flow measurements in southern Mexico are presented. These measurements document a very broad zone of low heat flow between the coastline and the Trans-Mexico volcanic belt. At the Trans-Mexico volcanic belt, heat flow increases to high values and remains high into central Mexico. Heat flow values in the Trans-Mexico volcanic belt and northward are in excess of 80 mW m-2 (1.9 μcal/cm2 s) and typical of values observed in the high heat flow regions of the western United States. The average heat flow in the Sierra Madre del Sur is 26 mW m-2 (0.6 μcal/cm2 s), in the Sierra Madre Oriental is 89 mW m-2 (2.1 μcal/cm2 s), and in the Trans-Mexico volcanic belt is 91 mW m-2 (2.2 μcal/cm2 s). A surface heat flow profile was constructed perpendicular to the trench, and the data were matched with a simple thermal subduction model. The resulting final model parameter set suggests that the angle of subduction is low and that convective heat transfer in the back arc region is probable. For data south of the Trans-Mexico volcanic belt, reduced heat flow values (upper crustal radioactive effect removed) are less than 15 mW m-2 (0.4 μcal/cm2 s). Thus essentially the total mantle heat flow is being absorbed by the subducting block. The values are lower than the heat flow observed in the Sierra Nevada Mountains of California and support the hypothesis that the heat flow couple associated with the Sierra Nevada basin and Range province is a relic subduction zone thermal pattern.