Abstract: Hydroxyaluminosilicate (HAS) and hydroxyaluminum (HyA) ionic solutions having final Al concentrations ranging from 3.74 to 4.00 mM; NaOH/Al molar ratios of 1.0, 2.0 and 2.5; and Si/Al molar ratios of 0.00, 0.27–0.30, 0.51–0.56 and 0.95–1.01 were prepared through the interaction of AlCl3, or-thosilicic acid and NaOH solutions. When these solutions reacted with <2 µm sized vermiculite (Vt) and montmorillonite (Mt), varying amounts of Al and Si were fixed on Vt and Mt clays. Potassium fixation and exchange capacities of HyA/HAS (OH/Al = 1.0, 2.0 and 2.5)-Vt and HyA/HAS (OH/AI = 2.0)-Mt complexes were compared with those of untreated Vt and Mt at added K levels ranging from 21 to 319 cmolc kg−1. The untreated Vt clay showed K fixation as high as 94 cmolc kg−1, in contrast to only 16 cmolc kg−1 exchangeable K. The untreated Mt fixed a maximum of 9 cmolc K kg−1 out of a total K adsorption capacity of 67 creole kg−1. In the HyA/HAS-Vt complexes, K fixation reduced drastically in comparison to untreated Vt, and ranged from 9 to 24 cmolc kg−1 out of their total K adsorption capacities of 61 to 81 cmolc kg−1. In the HyA/HAS-Mt complexes, too, the amount of K fixed reduced to a great extent in comparison to Mt and ranged from 1.48 to 1.84 cmolc kg−1. Potassium became more exchangeable due to the presence of hydroxy-interlayers in the clays. The reduction in CEC and the well-known propping effects of hydroxy-cations’ islands in the interlayers might have hindered K fixation by the complexes. The relationships of maximum K fixing capacities of the HyA/HAS-Vt complexes with the amounts of Al, Si and Al + Si fixed on Vt were all exponential and negative. However, the amount of Al + Si or only Al fixed on Vt appeared to be the best indicator of K fixation capacities of hydroxyinterlayered Vt clay.