The acceleration of13C nmr relaxation rates by oxygen or by nitroxide radical was determined for representative perfluorohexane-heptane mixtures. In the 90:10 (v/v) solution, oxygen is found to affect perfluorohexane, but on the other hand, nitroxide affects the alkane the more strongly. In 15∶:85 solutions, little differentiation was noticeable although the data are less secure. In the former solution, domains are believed to exist, which are similar to the pure solvent in ability to dissolve paramagnetic relaxants. Force field calculations on the shape of fluorocarbon molecules are reviewed.