Acidity and stability of activated carbon-based solid acid catalysts for aqueous-phase reactions are investigated. Carbon is acidified with liquid and gas phase methods, using nitric and sulfuric acid, hydrogen peroxide and calcination in air at 300 and 400 °C. Modified carbons are characterized by nitrogen physisorption, scanning electron microscopy (SEM), point of zero charge (PZC) measurements, Boehm titration, temperature-programmed desorption–mass spectrometry (TPD–MS), and X-ray photoelectron spectroscopy (XPS). Stability of acid functional groups under typical reaction conditions for biomass conversion is investigated by exposing carbons to hydrothermal treatment (i.e. hot liquid water). Special attention was devoted to elucidating the effect of the temperature (150–225 °C) and time of exposure (0–24 h) on the hydrothermal stability of different surface functional groups. Carbon modification by sulfuric acid generates strong acid sites in higher concentration, compared to carbon modified by nitric acid, calcination and hydrogen peroxide. In contrast to the other treatments, calcination at 400 °C increases carbon basicity. Although the concentrations of all surface acid sites decrease upon the hydrothermal treatment, this effect is not uniform. Stability of acid sites with different strengths and their chemical nature are dependent on the modification method. Strong acid sites formed by sulfuric acid treatment show a much higher stability than those formed by the other acidification procedures. The H2SO4-treated material retains ca. 40% of strong acid sites even after exposure to hot liquid water at 200 °C for more than 4 h. Only such strong acid sites remain on the carbon surface after exposure to hot liquid water at 225 °C.