A new MAS-NMR rotor (the WHiMS rotor) has been developed which can reach pressures of 400 bar at 20 °C or 225 bar at 250 °C. These rotors are ideal for mixed phase systems such as a reaction using a solid catalyst with a liquid/supercritical solvent topped with high pressure gas in the head space. After solid and liquid portions of the sample are loaded, the rotor is capped with an o-ring equipped polymer bushing that snaps into a mating groove in the rotor. The bushings incorporate a check valve into the sealing mechanism which allows for pressurization without mechanical manipulation – they will allow gas to flow in but not out. This WHiMS rotor design has enabled experiments on a wide variety of biotic and abiotic mixed-phase systems. Geochemical systems have also been studied, for example, adsorption and confinement studies of supercritical methane/CO2 in clays and other minerals which display pressure dependent 13C chemical shifts. Example data from other mixed-phase chemical and microbial systems are reported. These include monitoring metabolite conversion of extremophilic bacteria found in subsurface systems at elevated pressures and real-time operando reactions in catalysis systems - with liquid-quality resolution for 1H and 13C NMR spectra.