A method for the determination of the sulfonic acid content in lignosulfonates via sulfur dioxide quantification upon treatment with H3PO4 by headspace gas chromatography is presented and compared to two already established alternatives, conductometric titration and elemental analysis. Several lignosulfonates, purified from various industrial sources, were examined by all three methods. Limitations and possible interference by other functional groups, such as carboxylic acids, are discussed, and suitable solutions are presented. Results of the novel approach are comparable to those of the established techniques in terms of accuracy and precision. The limit of detection (LOD) is 0.88 μmol of SO3H and the corresponding limit of quantification (LOQ) is 3.78 μmol of SO3H. At the same time, this method outperforms conductometric titration in terms of a higher sample throughput and a much smaller sample amount needed for analysis. Treatment of the sample with highly concentrated phosphoric acid and simple heating is straightforward enough to render the method a valuable tool in lignosulfonate analysis of industrial problem sets, such as screenings or optimization of process parameters, without compromising analytical accuracy.