Ex situ room-temperature C-13 nuclear magnetic resonance (NMR) measurements are reported on powdered poplar wood that has been pretreated with dilute sulfuric acid (concentrations up to 1 wt %) for times ranging up to 20 mm and at temperatures of 120, 130, 140, and 150 degrees C. There are significant, albeit not dramatic, changes in the measured NMR spectra of the biomass as result of dilute sulfuric acid treatment. Values of T-1 for C-13 and as well as T-CH and T-1 rho H, were measured for lignin peaks and cellulose peaks in the C-13 NMR spectra, as potential indicators of the degree of atomic-level motion. For lignin components, one finds a trend to larger T-CH values as the treatment time or H2SO4 concentration is increased for treatment temperatures of 120 and 130 degrees C; however, for treatment temperatures of 140 and 150 degrees C, T-CH apparently decreases as the treatment time is increased. This higher temperature T-CH behavior implies that the lignin may actually become more rigid at later stages of treatment at temperatures >= 140 degrees C, which can be explained by cleavages of ether linkages of lignin and subsequent formation of new linkages, i.e., lignin recondensation. T-1C and T-1H measurements are consistent with this interpretation. The relationships between atomic-level mobility of lignin in biomass and treatment temperature is consistent with published relationships between the sugar yield and treatment temperature. The key role of acid treatment as a pretreatment for enzymatic digestion is evident in NMR measurements, including relaxation measurements, even after the treatment.