Abstract: A method for continuously measuring the concentration of CaCO.sub.3 in slurries comprising CaCO.sub.3 is described. The method comprises continuously sampling a given amount of the slurry, feeding the sampled slurry into an agitated continuous reactor container which is isolated from the outside air, keeping the slurry in the reactor container at a temperature of at least 50.degree. C., adding sulfuric acid or hydrochloric acid to adjust the pH to below 4, blowing a known flow rate of air into the slurry, withdrawing from the reactor container CO.sub.2, produced by the reaction between CaCO.sub.3 and the acid, by entrainment with the air, further mixing the withdrawn gas with air for dilution while controlling a flow rate of the dilution air so that the concentration of CO.sub.2 in the mixed gas is maintained constant, and calculating the concentration of CaCO.sub.3 in the slurry from the concentration of CO.sub.2 in the mixed gas, the flow rate of the mixed gas and the amount of the sampled slurry.