For potential applications as spin labels for DEER or polarization transfer agents in DNP NMR, nitroxides should also ideally exhibit sufficiently long relaxation times Tm to enable precise measurement of long distances, and efficient polarization transfer due signal enhancement, respectively. It is well- known that the presence of methyl groups shorten Tm due to methyl rotation, especially at temperatures above 70 K. Although the requirement for using cryogenic temperatures that rely on liquid helium increases the operating costs of EPR spectrometers, contemporary instruments now minimize liquid helium consumption by the use of circulating liquid helium pumps. Nevertheless, for reasons of cost and sustainability, it is desirable to preserve extended Tm into temperatures that can be acquired using liquid nitrogen, especially for DNP NMR applications. For Tm measurements, nitroxides were dissolved in 1:1 PBS buffer/glycerol, in which concentrations of 50 µM were easily achieved. After flash freezing in liquid nitrogen, samples were cooled to 50 K and spin echo decay curves were recorded using pulsed Q-band EPR spectroscopy. The spin echo decays of nitroxide cis,cis-9, 11, 12 and 13 at this temperature are shown in Figure . Dimethylated spirocyclohexyl nitroxide cis,cis-9 has a similar echo decay to the non-methylated spirocyclohexyl nitroxide 11, which exhibits the longest Tm. Tetramethyl nitroxide 12 has a slightly shorter Tm even at 50 K, but tetraethyl nitroxide 13 shows significantly more rapid relaxation. This has previously been ascribed to an increased local proton concentration in the tetraethyl substituents. However, we note that spirocyclohexyl nitroxides also have an increased local proton concentration, and that the line shape of the echo decay of 13 suggests there are multiple mechanisms of spin echo dephasing in operation for the tetraethyl system at 50 K.