Electron spin relaxation times of a Nycomed triarylmethyl radical (sym-trityl) in water, 1 : 1 water : glycerol, and 1 : 9 water : glycerol were measured at L-band, S-band, and X-band by pulsed EPR methods. In H 2 O solution, T 1 is 17 Β± 1 Β΅s at X-band at ambient temperature, is nearly independent of microwave frequency, and exhibits little dependence on viscosity. The temperature dependence of T 1 in 1 : 1 water : glycerol is characteristic of domination by a Raman process between 20 and 80 K. The increased spin-lattice relaxation rates at higher temperatures, including room temperature, are attributed to a local vibrational mode that modulates spin-orbit coupling. In H 2 O solution, T 2 is 11 Β± 1 Β΅s at X-band, increasing to 13 Β± 1 Β΅s at L-band. For more viscous solvent mixtures, T 2 is much shorter than T 1 and weakly frequency dependent, which indicates that incomplete motional averaging of hyperfine anisotropy makes a significant contribution to T 2 . In water and 1 : 1 water : glycerol solutions continuous wave EPR linewidths are not relaxation determined, but become relaxation determined in the higher viscosity 1 : 9 water : glycerol solutions. The Lorentzian component of the 250-MHz linewidths as a function of viscosity is in good agreement with T 2 -determined contributions to the linewidths at higher frequencies.