The sensitivity of the magnetic resonance experiment can be maximized by keeping the effective volume of the sample resonator as small as possible, and by making Qo, the quality factor of the unloaded resonator, as large as possible (I). We describe here a design in which a cylindrical dielectric resonator is used in place of a TElo2 metalwalled cavity for electron paramagnetic resonance studies. The dielectric resonator has a volume of 0.075 cm3 and a Qo of 5300 (2). The Teloz cavity volume is 10 cm3 with a Qo of 7000. Therefore, the resonator volume has been reduced by a factor of 133 while maintaining alarge Qo value. Comparing results from the dielectric resonator and the cavity shows that a remarkable increase in EPR spectrometer sensitivity has been achieved. For saturable samples of limited size, the dielectric sample resonator yields up to 200 times greater signal than the cavity, and for limited size unsaturable samples (such as aqueous Mn*+) up to 150 times greater signal intensity is realized. In 1939, it was considered theoretically possible that suitably shaped objects made of a dielectric material could function as electrical resonators at high frequencies (3). In the 1960s a few researchers (4-6) used rectangular pieces of ceramic rutile (Ti02) placed in a waveguide for EPR studies. These experiments were simplified by using t-utile resonators doped with paramagnetic impurities such as Fe and Cr. A rutile resonator was also used to observe an EPR signal of DPPH which surrounded the resonator in a thin layer (4). This latter technique is possible because the microwave magnetic field extends outside the resonator for a short distance (3). These results indicated that a dielectric resonator could replace a metal-walled cavity for EPR investigations. However, until the late 1970s resonator applications were limited by the lack of suitable materials which possessed acceptable values of Qo, dielectric constant, and temperature coefficient of resonant frequency (7). Materials such as Ti02 which had a Qo of 10,000 at 4 GHz, and a dielectric constant of 100 were often used for exploratory work, but Ti02 has a temperature coefficient of resonant frequency of 400-1000 ppm/“C (4, 7). In many ways, a dielectric resonator is comparable in characteristics but much smaller in size than a metallic resonant cavity. The electric short circuit boundaries (metal walls) of the cavity are replaced by magnetic short circuit boundaries (dielectric walls) of the dielectric resonator. It can exist in any regular geometric form and resonates in various modes at frequencies determined by its dimensions, dielectric constant, and surrounding shielding conditions (7). For our sample resonators we used a com-