Electron spin resonance (ESR) studies were carried out for 2mM 14N-labeled deutrated 3-methoxycarbonyl-2,2,5,5-tetramethyl- pyrrolidine-1-oxyl (MC-PROXYL) and 3-carboxy-2,2,5,5,-tetramethyl-1- pyrrolidinyloxy (carboxy-PROXYL) in pure water and various concentrations of liposomal solutions by using 300 MHz ESR spectrometer. The ESR parameters such as the line width, hyperfine coupling constant, g-factor, rotational correlation time, partition parameter and permeability were reported for the samples. The permeable and impermeable nature of nitroxyl spin probes was demonstrated. The increased rotational correlation time was observed for MC-PROXYL, which indicates the permeable and less mobile nature of the nitroxyl spin probe. The obtained partition parameter and permeability values are used to quantify the permeable nature of the nitroxyl spin probe. The peaks correspond to lipid phase were observed for MC-PROXYL in liposomal solution, but that peak was not observed for carboxy-PROXYL. These results indicate the permeable and the impermeable nature of nitroxyl spin probes.