Methanesulfonic acid (MSA) is often employed as a solvent for high-performance polymers such as poly(p-phenylene-cis-benzobisoxazole) or PBO, aramides, and conducting polymers. Since MSA is highly hygroscopic, and water has very strong effects on the physicochemical properties of the acid, it is important to develop methods to control and assess its composition. An approach involving the use of methanesulfonic anhydride (MSAA) is presented. The properties and compositions of MSAA solutions in MSA were investigated by combining H-1 NMR, conductimetry, viscometry, and differential refractometry. The results showed that previous studies involving MSA polymer solutions, and reported physical properties of MSA, were affected by water contamination. New values for the refractive index, viscosity, and specific conductivity of water-free MSA are presented.