A nuclear magnetic resonance (NMR) probe for high pressure, high temperature studies is presented. While applicable to many physical systems, the device is optimized for the study of the physics and chemistry of supercritical water and its solutions. The design is modular and is particularly simple, using readily available parts and materials. A new approach is presented for elimination of the magnetic field from the heater currents. The probe has been used to 600 °C and 400 bar. The rf performance is quite good; the NMR linewidth is about 0.1 ppm full width at half-height at any pressure and temperature
