The complex permittivity of milk at room temperature (17-20 degrees C) was studied over the frequency range of 1-20 GHz. UHT whole, low fat and skim milk were examined, fresh from the container, and over a period of two weeks while they were allowed to spoil at room temperature. In addition, the spectrum of whole milk at various dilutions was also measured. Both the real and imaginary parts of the permittivity were found to be smooth, slowly varying functions of frequency over this range. The Debye relation, with an additional term for ionic conduction losses was fitted to the data, and six parameters were extracted. The variation of these parameters with fat content and dilution suggests that they may be useful to roughly determine the milk's content in terms of groups of materials (ionic compounds, fats, and carbohydrates and proteins). Spectra also vary significantly with spoilage, but analysis is complicated by concerns of physical (phase separation) in addition to chemical changes. (c) 2005 Elsevier Ltd. All rights reserved.