Describes a new, non-destructive method to measure the sulfation grade of lithoid and stonework surfaces. An exchange between ammonium carbonate and calcium sulfate, in the presence of paper or strong and weak anionic exchange resins allows for a determination of the whole quantity of SO(4) ions which are on surfaces of some cm(2), within 30 minutes or some hours (depending on sulfation intensity). Now tested in tens of interventions on lithoid monuments and frescoes, the method provides an indication of the chromatic effect of desulfation.