High cation-exchange capacity (CEC) muscovite mica with a homoionic surface was prepared by replacing the Li+ surface ions of partially delaminated Li-mica with K+. The CEC of this K-mica was determined by exchanging its surface cations with Cs+, NR4+, methylene blue (MB+), and copper triethylenetetramine [Cu(trien)(2+)]. The kinetics of these exchange reactions were studied and showed large differences depending on their relative affinities to mica. The NH4+/K+ exchange was slow, while the Cs+ and Cu(trien)(2+)/K+ exchange was fast. The MB+/K+ exchange was quite slow and was not completed even after 99 h. Insufficient reaction time is one of the main reasons for the contradictory results reported in the literature for the CEC of aluminosilicates obtained by different methods, The CEC of mica can be photometrically measured by exchanging its surface cations with Cu(trien)(2+)