A geothermal study of western Spain based on water; mining, oil and geothermal boreholes with a total of 167 thermal gradient measurements, 85 thermal conductivity measurements and 640 radiogenic heat production measurements is presented. Many thermometriesare affected by groundwater circulation, particularly in the Betic mountain belt where it was impossible to define regional gradient values because of the high scattering of data. The measures define two areas thermally differentiated in the Hesperic Massif: the northern part which correspondes to the Central Iberian zone and the southern part which correspondes to the South-Portuguese zone. Average heat flow for metamorphic rocks is 20% higher in the South-Portuguese zone, This fact could be explained either by: a thinner lithosphere or by a higher radiogenic heat production in the South-Portuguese zone, or by a combination of both. We suggest that the thickness o f the th e rm a l lith o sphereis rather constant ( 1 1 0 ± 1 0 km) in the Hesperic Massif
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