A detailed theoretical investigation of the tetramethylammonium(TMA)-benzene and TMA-pyrrole complexes has been performed to obtain the interaction properties of TMA with aromatics. Diffuse :functions have been found to be important in the computational studies of these noncovalent complexes. Adding diffuse functions to the basis set decreases the binding energy by about 10% for the TMA-aromatic systems. Dispersion interactions in the TMA-aromatic systems are very important. They enhance the binding interactions between the TMA and the aromatic ring systems by about 0.5 kcal . mol(-1) per interacting atomic pair, which is in agreement with the estimates of Rappe and Bernstein.(1) Also, for the TMA-pyrrole complex, the presence of the dispersion interaction leads to a dramatic change in the optimized structure. Because B3LYP cannot handle properly the dispersion in the calculation, use of the Moller-Plesset second-order perturbation or other sophisticated methods should be considered in computational studies of cation-pi interactions in systems containing nonsymmetric dispersion interacted atomic pairs. The orbital interaction is unimportant in the TMA-aromatic interaction according to the detailed analysis of the molecular orbitals. The TMA-aromatic interactions basically come from the typical cation-pi interaction and the dispersion interaction. Because the electron density in the II56 aromatic system of pyrrole is larger than that in the II66 system of benzene, the pi electron cloud on pyrrole is more easily polarized under the influence of cations, which may lead to a relatively stronger cation-pi interaction in the TMA-pyrrole complex than in the TMA-benzene complex.