Sulphonic acid analyses were performed on wing refiner chemithermomechanical pulps, low sulphite charge pulps, thermomechanical pulps and on wood samples using a new method referred to as the niacin method. This method gives sulphonic acid content comparable to those estimated with the quinoline method or the Schoniger method. The method gives low interference with carboxylic acids and dissolved substances in the pulp if the pH is kept sufficiently low and the pulp is well washed.