Density functional theory was used to study the structure and conformational equilibrium of tetraaminothiacalix[4]arene (1) and tetramercaptothiacalix[4]arene (2), which are expected to open a new area of host-guest chemistry. All the calculations predict the cone as the most stable conformer for both compounds, which is in good agreement with an earlier theoretical study on the parent thiacalix[4]arene. It is shown that different substituents at the lower rim do not affect the stability ordering. The intramolecular bonding between hydrogen atoms and sulfur bridges seems to be a dominant factor in stabilizing all the conformers rather than hydrogen bonds between the groups of the lower rim. (C) 2004 Elsevier B.V. All rights reserved.