EPR line shapes in fluid solution and electron spin relaxation times and lineshapes in frozen solution were measured at X-band for two dinitroxides. For dinitroxide 1, which has two proxyl nitroxides connected by an amide linkage, the fluid solution spectra are in the strong exchange limit (J >> nitrogen hyperfine coupling) and the interspin distance in glassy water:glycerol is about 8.8 A. For dinitroxide 2, which has two proxyl nitroxides connected by a bridging ethylenediamine and two amide linkages, the fluid solution spectra are characteristic of dynamic exchange between conformations with strong exchange (J > 850 MHz) and conformations with weaker exchange interaction. For 2, the interspin distance in glassy water:glycerol is about 9.2 A and J > 600 MHz. T1 and Tm for both dinitroxides in 1:1 water:glycerol were measured at 10 to 160 K. Relaxation times are within experimental uncertainty of values for a pyrrolinoxyl monoradical and very similar to values for a nitroxide with a piperidinyl ring. These data demonstrate that even for these relatively flexible linkages dipolar interaction with an interspin distance of about 9 A does not provide an effective spin–lattice relaxation mechanism for dinitroxides in glassy water:glycerol.