We announce a new technique for the detection of changes in the conformation of small globular proteins in solution. We employ a coaxial-fed slot antenna with resonant frequencies in the 2-5 GHz range. This antenna detects changes in the dielectric properties of water. All proteins are surrounded by one or more shells of bound water. The dielectric properties of this 'bound' water are distinguishable from those of bulk water. As a protein changes it conformation, complementary changes occur in the three-dimensional arrangement of the 'bound' water. Thus, water can be used as a reporter for changes in protein conformation. Our technique has two advantages over conventional methods for microwave spectroscopy. First, unlike time-domain dielectric spectroscopy, data is measured in the frequency domain, so that time-to-frequency conversions are not necessary. Second, slot antennas may be affixed to the exterior of conventional fused-quartz cuvettes, so that simultane-ous measurements can be obtained using the antenna and conventional optical methods such as UV/VIS spectroscopy. When the unfolding of bovine pancreatic ribonuclease A (RNase A) is monitored at microwave frequencies, peak shifts in the antenna's resonant frequencies reflect changes in the protein's conformation. These peak shifts are sigmoidal with respect to temperature, and fit well to a two-state reversible unfolding model. Such sigmoidal peak shifts are not present when non-protein solutions are heated.