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The use of radiometers for biomedical applications needs a coherent understanding of thermal signals emitted by the tissues. In this paper, we show first, that some precautions have to be taken when measuring a temperature by the classical radiometric method. For instance, the signal emitted by a lossy material depends on its temperature, permittivity and thickness. This remark allows us to find out a new method for measuring microwave permittivity of liquids when using a Dicke radiometer. We propose a modified radiometric method to determine directly the temperature of the material whatever its reflection coefficient. The applicability of this method is tested with a X band set-up including FET microwave amplifiers. Possibilities of using probes for in situ temperature measurements are discussed.