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Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan [delta]). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables. Index Terms--Dielectric response, Karl Fischer titration, mass impregnated cable, moisture content, oil paper insulation.