A systematic analysis of the mode structure of diffusive relaxations in 1 MDa hydroxypropylcellulose(HPC):water is presented. New methods and data include (1) use of integral spectral moments to characterize nonexponential decays, (2) spectra of small probes in concentrated HPC solutions, (3) temperature dependence of the mode structure, and (4) comparison of optical probe spectra and spectra of probe-free polymer solutions. We find that (1) probe and polymer relaxations are in general not the same; (2) the apparent viscometric crossover near c(t)approximate to6 g/l is echoed by probe behavior; (3) our HPC solutions have a characteristic dynamic length, namely the 50 nm length that matches the polymer's hydrodynamic radius; (4) characterization of spectral modes with their mean relaxation time affords simplifications relative to other characterizations; and (5) contrary to some expectations, Stokes-Einsteinian behavior (diffusion rate determined by the macroscopic viscosity) is not observed, even for large probes in relatively concentrated solutions. We propose that the viscometric and light scattering effects found in HPC solutions at elevated concentrations reflect the incipient formation of a generalized Kivelson [S. A. Kivelson , J. Chem. Phys. 101, 2391 (1994)] glass. (C) 2003 American Institute of Physics.