EPR spectra of the nitroxide radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) are reported in super critical and in liquid n-hexane and ethanol for 25-200 bar and 295-600 K. At low radical concentration (<10(-5) M) the line width is controlled by the spin-rotation interaction and is quantitatively predicted by liquid-phase theory for solvent densities rho greater than the critical density rho(c). Deviations at lower densities are assigned to clustering effects and the crossover to gas-phase behavior. Rate constants for Heisenberg spin exchange ([TEMPO] > 10(-3) M) up to 580 K are also reported. They again follow liquid-phase theory for rho > rho(c). Deviations at low densities may be explained by a gas-phase behavior without clustering effects.