The density dependences of the nitrogen hyperfine coupling constant (a(N)) and line width (DeltaH(pp)) of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) free radical have been investigated at near-infinite dilution in gaseous and supercritical carbon dioxide using steady-state EPR spectroscopy. The local density augmentation around the radical is quantified based on the density dependences of the a(N). The viscosity dependences of the DeltaH(pp) are reproduced by the modified Stokes-Einstein equation, suggesting the local solute-fluid density augmentation. (C) 2003 Elsevier B.V. All rights reserved.