The in situ photolytic exchange of ethylene and hydrogen for carbon monoxide on cymantrene (CpMn(CO)(3), Cp = eta(5)-cyclopentadienyl) and methylcymantrene (MeCpMn(CO)(3), MeCp = monomethyl-eta(5)-cyclopentadienyl) dissolved in subcritical and supercritical solvents (CO2 and ethylene) was investigated by high-resolution, high-pressure H-1 NMR over the temperature range from -40 to 150 degrees C and a pressure range from 35 to 2600 bar. Photolytic substitution of ethylene for CO proceeded to completion under all conditions investigated, and only one ethylene was observed to substitute for CO on the manganese complexes even in neat ethylene under extreme conditions of pressure and temperature. Only small amounts of dihydrogen were observed to substitute for CO on cymantrene at 35 degrees C in a binary solvent mixture of CO2/H-2 during photolysis. The H-1 chemical shifts of the manganese complexes and their ethylene substituted products were found to be linearly dependent on density with temperature and solvent dependence also observed, The spin-lattice relaxation times (T-1) of all the solvent and solute species were observed to be inversely proportional to the density of the solvent over the range of conditions investigated, Temperature and concentration dependent phase behavior and solute saturation were also determined for the methylcymantrene and MeCpMn(CO)(2)(eta(2)-C2H4) solutes in SCF ethylene. These results represent the first NMR detected in situ photolysis study of organometallics in SCF and demonstrate the utility of this technique.