In the presence of benzene, naphthalene, or phenanthrene, the NH stretching band of betacarboline in tetrachloroethane diminishes in intensity while a new red-shifted band grows. These changes are attributed to the formation of molecular association complexes between betacarboline and the benzenoid compounds. Deconvolution of these IR bands allowed us to calculate their association constants, which increase linearly with the number of pi electrons of the benzenoid substrates. The shifts of the associated bands were, however, independent of the bases. These results suggest that the complexes are stabilized by the hydrogen-bonding interaction between the pyrrolic NH group of betacarboline and the pi-delocalized electrons of the benzenoid substrates. Low-level semiempirical calculations (AM1/MOPAC) also predict T-shaped structures as the most stable. On the other hand, experiments using harmane, the 1-methylated betacarboline, proved steric hindrance for naphthalene and phenanthrene, which should be a consequence of the geometry of the complexes.