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This paper proposes a technique for determining the distributions of the electric field and space charge in the insulation of power cables by using the data acquired with the thermal step method (TSM). The TSM consists of applying a low-temperature step to a short-circuited or dc-energized cable and of acquiring a transient capacitive current. The processing technique described in this paper is based on a series decomposition of the electric field, the coefficients of the series being identified via the measured current. The accuracy and the stability of the calculation method are evaluated by simulations performed using various distributions of the electric field and different noise levels. An application of the technique to space charge measurements in a dc conditioned power cable is then presented.