Geothermal energy sources are usually associated with volcanic regions where hot water springs and vapour emanation open up large areas of interest. The final evaluation of a possible worthwhile area for electric power production is carried out by drilling deep wells to measure in them well-known parameters to determine the potential of the well. Since drilling such wells is very expensive, it is necessary to reduce their number to a minimum by studying the area prior to drilling. Amongst the studies that contribute to useful information is radon mapping of the areas of interest. Interpretation of these results and other data provided be geophysical survey limit the number of boreholes required. This paper describes radon mapping by means of registering α-particles in thin-plastic Solid State Nuclear Track Detectors. Studies were made in four Mexican geothermal fields for the National Agency for Electric Power Supply (CFE).