Structures of hectorites and saponites intercalated with a Keggin like cation [Al13O4(OH)(24)(H2O)(12)](7+) have been studied using molecular simulations in a Cerius(2) modelling environment. Present study is focused to the effect of octahedral and tetrahedral substitutions in the silicate layers on the crystal packing and interlayer porosity of intercalated trioctahedral smectites. Translation of the Keggin cation along the silicate layers results in only small fluctuations of basal spacings and energy values. No correlations were found between the interlayer position of the shifting Keggin ion and the crystal energy values for the hectorite structure because of its even charge distribution. However the interlayer translation of the Keggin ion shows more distinct, preferential positions with lower energy values for the saponite structure due to its tetrahedral substitution. Consequently a more homogeneous distribution of the Keggin ion positions is more probable for pillared saponite. This more homogeneous distribution of the pillaring cation leads to a better porosity control for pillared saponites compared to pillared hectorites.