We present the results of three detailed heat flow surveys which are used to investigate the variations of heat flow and age of the Balearic and Tyrrhenian basins in the western Mediterranean. Analysis of 12 measurements within a 10-km radius from 40¿01'N, 4¿55'E in the Balearic abyssal pain shows a mean heat fflow of 92¿10 mW m-2. After correction for the effects of sedimentation this value agrees well with the predictions of plate cooling and high extension stretching models for crust of the late Oligocene age proposed for the basin. A similar survey with 18 measurements around 40¿16'N, 11¿19'E in the Western Tyrrhenian gives a flux of 134¿8 mW m-2, while the third survey of 26 measurements in the southern Tyrrhenian abyssal plain at 37¿17'N, 12¿58'E yields a slightly higher heat flow of 151¿10 mW m-2. These values are within the range predicted by simple plate cooling models for the late Miocene ages of the deep Tyrrhenian basin. Thus our observations suggest that although the mode of crustal formation of these deep marginal basins is less well defined than that of the major ocean basins, the thermal signature is similar. Also, the trend of increasing heat flow from west to east through the Balearic and Tyrrhenian basins is in agreement with models of the formation of the Western Mediterranean behind an eastwardly migrating trench system. In all three areas the measured flux shows significant local variability. In the two westernmost surveys this can be attributed to the presence of buried, high-conductivity salt structures, but in the third area (SE Tyrrhenian) the magnitude of the variations is several times greater than can be attributed to steady state thermal refraction alone. Instead, evidence exists for localized hydrothermal activity linked to the small topographical relief within the survey area.