Due to the interest in the non-invasive measurement of subcutaneous temperature, a four-frequency microwave radiometer system has been tested on a two-dimensional thermal phantom consisting of a hotter tube embedded eccentrically in a colder circular container simulating a cylindrical body region with an inhomogeneous temperature distribution. It is shown that measured radiometric data agree with a theoretical model of the electromagnetic radiation from thermal structures. An equation for retrieving temperature distributions in homogeneous circular cylinders has been obtained by taking regularity properties of temperature functions into account. Results of temperature reconstructions from both measured and synthetic data are reported showing the capability of the method.