The microscopic structure of frozen aqueous sucrose solutions, over concentrations of 0-75% (w/v), is characterized by using multiple continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopic and relaxation techniques and the paramagnetic spin probe, TEMPOL. The temperature dependence of the TEMPOL EPR line-shape anisotropy reveals a mobility transition, specified at 205 K in pure water and 255 5 K for >1% (w/v) added sucrose. The transition temperature is >> T-g, where T-g is the homogeneous water glass transition temperature, which shows that TEMPOL resides in the mesoscopic domain (mesodomain) at water ice crystallite boundaries and that the mesodomain sucrose concentrations are comparable at >1% (w/v) added sucrose. Electron spin-echo envelope modulation (ESEEM) spectroscopy of TEMPOL-H-2(2)-sucrose hyperfine interactions also indicates comparable sucrose concentrations in mesodomains at >1% (w/v) added sucrose. Electron spin-echo (ESE) detected longitudinal and phase memory relaxation times (T-1 and T-M, respectively) at 6 K indicate a general trend of increased mesodomain volume with added sucrose, in three stages: 1-15,20-50, and >50% (w/v). The calibrated TEMPOL concentrations indicate that the mesodomain volume is less than the predicted maximally freeze-concentrated value [80 (w/w); 120% (w/v)], with transitions at 15-20% and 50% (w/v) starting sucrose. An ordered sucrose hydrate phase, which excludes TEMPOL, and a disordered, amorphous sucrose water glass phase, in which TEMPOL resides, are proposed to compose a heterogeneous mesodomain. The results show that the ratio of ordered and disordered volume fractions in the mesodomain is exquisitely sensitive to the starting sucrose concentration.