Microwaves can be used to detect hidden objects behind optically opaque materials. Hence, the penetration capability through such materials is of fundamental importance. In order to characterise a material of interest in the microwave region, its permittivity should be known besides its physical structure. In many cases the permittivity is unknown, inaccurately known, or known for only specific frequencies. Also very often the range of values given in the literature can have a large variability for a specific situation. In this paper we describe a procedure to determine the permittivity from radiometric free-space measurements of nearly arbitrary materials. The advantage of this method is that large material samples like brick or wooden plates, and materials like textiles, which are hard to mount in a defined way in a waveguide, can be investigated. Some representative results for MMW measurements are shown, and an estimation of the presently achieved precision is given. The first attempts showed a satisfying performance, although not for all materials and frequencies a unique solution could be found.