Hydrothermal stability of carbon based acid catalysts synthesized by sulfonating carbohydrates pyrolyzed at moderate temperatures (300–600 °C) has been reported previously. To test the effect of carbon structure on hydrothermal stability, we produced catalysts by dry pyrolysis at 350 °C and 450 °C or by hydrothermal carbonization, followed by sulfonation with fuming sulfuric acid, as well as by direct sulfonation of glucose. The catalysts were characterized by BET, titration, Raman spectroscopy, TGA, XPS, reaction testing, and 13C solid state NMR. Catalysts were hydrothermally treated and then analyzed for sulfur retention and catalytic activity. The lower temperature carbon catalysts showed the best stability, however all showed significant activity loss. Solid state NMR of materials made from 13C-glucose was used to characterize the structural details in an attempt to correlate functional groups to hydrothermal stability of catalyst active sites. Structural models generated from NMR data showed that the most stable catalysts contained a significant fraction of furan rings and hardly any polycondensed aromatic rings.