Corncob and Miscanthus feedstocks were hydrothermally carbonized at 230 degrees C for 6 h in an autoclave under autogenous pressure, obtaining fine brown powders with higher carbon and lower hydrogen and oxygen contents than the starting materials. The chemical structure of feedstocks and hydrochars was investigated by C-13 cross-polarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) spectroscopy complemented by Fourier transform infrared (FTIR) spectroscopy and elemental analysis. In particular, a procedure including CP dynamics analysis and spectral deconvolution was applied to obtain quantitative information on the composition of the analyzed materials from C-13 CP-MAS spectra. Miscanthus feedstock contained larger amounts of lignin and crystalline cellulose, which concurred to reduce cellulose exposition to hot water during the hydrothermal carbonization (HTC) process. As a consequence, a lower furan/aromatic ratio was found for Miscanthus versus corncob hydrochars.