One of the most investigated heat transfer fluid or heat storage medium in the field of thermal solar conversion is the "solar salt", a mixture of nitrates NaNO(3)/KNO(3), 60/40% by weight, respectively. The content of potassium in this fluid allows exploiting the self activity of the isotope (40)K to perform measurements useful for control or diagnostics of a solar plant. Here an experimental test on the non-invasive measurement of the salt level in a tank by means of gamma-ray spectrometry is shown. (C) 2008 Elsevier Ltd. All rights reserved.