A solution or slurry containing a carbonate or sulfite, or both is continuously supplied into a tightly closed vessel, while any excess of the liquid is allowed to overflow the vessel. Sulfuric acid is supplied into the vessel to maintain the pH level of the liquid in the vessel at a value not exceeding 3. Air is supplied into the vessel as a carrier gas for CO.sub.2 or SO.sub.2 or both formed by the reaction of the carbonate or sulfite or both in the liquid with the sulfuric acid. The carrier gas is collected from the vessel, and the concentration of CO.sub.2 or SO.sub.2 or both in the carrier gas is determined. The concentration of the carbonate or sulfite or both in the liquid is obtained by calculations from the concentration of CO.sub.2 or SO.sub.2 or both, the flow rate of the liquid and the flow rate of the carrier gas.