Interactions involving the amino group (R-NH2) are weaker than those of alcohols (R-OH), and weaker intermolecular forces expand the isomerization alternatives for molecular aggregation. As an example, we observed three isomers in the prototype model of the aniline trimer (An3). For comparison, only one isomer was observed in the phenol trimer. This work was conducted in collaboration with Brooks H. Pate at the University of Virginia. In the homotrimer the weak N-H···N hydrogen bonds are assisted by subtle combinations of N-H···π and C-H···π interactions, producing several competing low-lying ring species in the gas phase. One of the aniline trimers is a symmetric top, topologically equivalent to the only observed phenol trimer.