Klason lignins lab-isolated from two lignocellulosic biomasses emerging from the processing of agro-industrial products, Aspidosperma australe wood sawdust and Bertholletia excelsa nutshells, with different contents of lignin (27 and 57 wt%, respectively), are characterized. Characterization includes determination of ash content, elemental composition, Fourier-transform infrared spectra, and surface morphological features by scanning electronic microscopy. Pyrolysis behavior of the isolated lignins as well as of the parent biomasses from which they are extracted is comparatively examined by non-isothermal thermogravimetric analysis from room temperature up to 1000 degrees C. The lignins exhibit different ash contents and elemental compositions, as well as noticeable differences in their pyrolytic behavior depending on the botanical origin of the bio-resource and with respect to that determined for the untreated parent biomasses. Overall, the lignin isolated from the nutshells is more resistant to degrade than the sawdust-derived lignin, likely due to a more condensed chemical structure.