Proton ENDOR spectra of the spin-label 2,2,5,5-tetramethyl-1-oxypyrroline-3-carboxylic acid in methanol, chloroform/toluene, and toluene exhibited well resolved features, permitting accurate estimation of hyperfine coupling (hfc) components of the vinylic and methyl protons. The magnitude and relative sign of the isotropic hfc A(iso) for these two classes of protons were determined by electron nuclear double resonance (ENDOR) and TRIPLE spectroscopy. While the isotropic hfc of methyl protons was temperature independent, A(iso) of the vinylic proton was temperature dependent in fluid and frozen solutions. The temperature dependence was not influenced by solvent polarity, but at T less than or equal to 50 K, A(iso) of the vinylic proton became temperature independent. The temperature dependence is explained on the basis of the out-of-plane vibration of the vinylic proton. Above 50 K, the out-of-plane vibration of the C-H-V bond causes the vinylic proton to become displaced from the nodal plane. The ratio of the measured A(iso) values of the vinylic proton and deuteron indicated that the vibrational mechanism is responsible for the temperature dependent isotropic hfc of the vinyl proton of the oxypyrrolinyl nitroxyl spin-labels.