A quantitative model of carbonate formation in the effluent of one-stage oxygen delignification for a softwood kraft pulp was developed and experimentally validated. This model quantitatively described carbonate formation in the effluent of oxygen delignified SW kraft pulps and its kinetic characteristics. It could be used as an approach to predict pulp kappa numbers in the solid phase (fiber) from carbonate content in the liquid phase (filtrate). From the kinetic behavior of carbonate formation and its relationship to pulp kappa number, a control strategy was suggested. Constant kappa number curves used for parameter selection were developed to achieve target pulp kappa number while retaining pulp viscosity and total fiber charge properties. © 2007 American Institute of Chemical Engineers AIChE J, 2007.