Abstract: Saponites were hydrothermally grown in the presence of amounts of NH4+, Na+, K+, Rb+, Ca2+, Ba2+, and Ce4+ equivalent with the CEC of the saponite (155 meq/100 g), with or without F-, at a temperature of 200°C for 72 hr. XRD and CEC data revealed the formation of a two-water-layer saponite with mainly Mg2+ as interlayer cation. Dehydration occurred between 25° and 450°C and dehydroxylation occurred in two steps between 450° and 790°C and between 790° and 890°C. The relatively small length of the b-axis between 9.151 and 9.180 Å is explained by considerable octahedral Al substitution (between 0.28 and 0.70 per three sites) and minor tetrahedral Al substitution (between 0.28 and 0.58 per four sites). Under the synthesis conditions applied in this study, less than 13% of the interlayer sites are occupied by Na+, K+, and Rb+; between 13.3% and 21% by Ca2+ and Ba2+; while NH4+ gives the highest value at 34%. The remaining sites are mainly filled by Mg2+. Ce4+ is not found in the saponite structure due to the formation of cerianite, CeO2. The presence of F- had little influence on the saponite composition. The formation of Mg-saponites is explained by a model in which an increased bayerite formation resulting in a higher octahedral Al3+ substitution and more Mg2+ in solution. Mg2+ is preferentially incorporated compared with the other interlayer cations due to its smallest ionic radius in combination with its 2+ charge.