The mobility of solutes in frozen pork (longissimus dorsi) was studied by X band electron spin resonance spectroscopy (ESR) using the three water soluble spin probes TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), CAT 1 (4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl) and TEMPO choline (4-(N,N-dimethyl-N-(2-hydroxyethyl))ammonium-2,2,6,6-tetramethylpiperidine-1 -oxyl). The mobility was quantified as the observed hyperfine coupling constant A' z z . The three spin probes, of which only the neutral TEMPOL may penetrate membranes, gave very similar values of A' z z as a function of the temperature. A' z z was constant at temperatures below -55 o C, whereas A' z z decreased as the temperature was increased above -55 o C. This behavior has been interpreted as an increase in the amplitude of the rotational oscillations of the spin probes (and other solutes) with temperature. Above -13 o C the spin probes gave isotropic ESR spectra indicating free rotational mobility in a molten liquid phase. These results are discussed in relation to glass transitions in frozen pork.