Some new high-pressure NMR techniques for in situ studies of homogeneous catalytic processes are described. High-pressure probes that incorporate toroid detectors for optimal sensitivity were employed to investigate the oxo reaction for the catalytic hydroformylation of olefins in supercritical carbon dioxide solution. The in situ measurements required pressures as high as 500 atm at 250 °C. Thermodynamics for the dissolution of the oxo catalyst, CO₂(CO)₈, in supercritical CO₂ and for the reactions of CO₂(CO)₈ and Mn₂(CO)₁₀ with hydrogen in this medium were determined using ⁵⁹Co, ⁵⁵Mn, and ¹H NMR spectral measurements. Reaction rates and steady-state concentrations of catalytic species in the hydroformylation of propylene in supercritical CO₂ were found to be comparable to those measured for typical liquid hydroformylation media. However, because only one phase is present in the supercritical system, stirring to achieve gas-liquid mixing was unnecessary, and an improved yield of linear product was achieved in CO₂, even without stirring. Processes involving the tetracarbonylcobalt radical, ^.Co(CO)₄, dominated the NMR observable high-temperature chemistry of the oxo catalyst. Activation barriers for hydrogen atom transfers and CO exchange involving this radical were measured by dynamic NMR line-shape analysis and contact shift NMR techniques. Thermodynamics for the dissociation of CO₂(CO)₈ into two ^.Co(CO)₄ radicals were measured using NMR magnetic susceptibility determinations in supercritical CO₂. In other research, the powerful radio frequency field gradient within a toroid cavity detector was utilized for the first time to achieve NMR microscopy capabilities in a high-pressure NMR probe. The toroid cavity imager provides NMR structural information as a function of radial distance from the central conductor in the torus with a resolution of a few micrometers. In a third research activity, high-pressure NMR measurements were used to detect the activation of methane using a soluble metal-metal bonded rhodium phthalocyanine dimeric complex, bis[(octa-n-pentylphthalocyanatorhodium(II)]. With this complex, methane is activated by means of the rhodium center`s ability to form strong bonds with both carbon and hydrogen. The methylrhodium and hydridorhodium products of the activation were characterized by NMR .and x-ray crystallographic methods.