Monolayers of 4-tetradecaneamido-2,2,6,6-tetramethyl-1-piperidinyloxy (C14TEMPO) were investigated at the air/water interface. Pressure-area diagrams and Brewster angle microscopy provided evidence of supercritical state of this monolayer at temperatures as low as 2 degreesC. The supercritical character of C14TEMPO Langmuir monolayers allowed the 2-D voltammetric measurements of its lateral diffusion constant, D, to be extended into a previously inaccessible region of low surface densities with mean molecular areas A similar to 600 Angstrom (2)/molecule. At A > 250 Angstrom (2)/molecule, D becomes independent of the C14TEMPO surface concentration and depends solely on the hydrodynamic coupling of the polar headgroup to the aqueous subphase. These measurements and the supercritical state of C14TEMPO monolayer open a possibility to probe the viscoelastic properties of the water liquid-vapor interfacial region.