A method of monitoring CaCO3 production from absorption of CO2 into an aqueous solution of Ca(OH)(2) is described. Its application to the measurement of Ca(OH)(2) conversion across different radial zones on a spinning disc reactor is discussed and results from experiments are presented. Links between conversion and models based on diffusive mass transfer into a thin rotating film are examined and a correlation with Fourier and Reynolds numbers shown. Estimations of mass-transfer coefficients based on the experimental results are provided and comparisons made with previous work. (c) 2005 American Institute of Chemical Engineers.