A numerical method developed by Versteeg (1989, Chem. Engng Sci.44, 2295–2310; 1990, Chem. Engng Sci.45, in press) is applied to the absorption of Co2 into solutions of blends amines. The experimental results of Critchfield and Rochelle (1987) are evaluated with the numerical model. It is shown that a rigorous numerical solution of the differential equations describing the mass transfer gives more insight into the actual process than analytical approximations based on a reduction of the number of reactions by neglecting or lumping reactions.