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Reaction rates and equilibrium constants of indole dimerization and trimerization in aqueous sulfuric acid at 298 K are reported. The equilibrium of oligomerization is attained in about 4–5 h, and formation of oligomers with more than three monomeric unit is not observed. The equilibrium of formation of the indole dimer is influenced by the protonation equilibrium of indole, which means the p K IH values of indole strongly influences equilibria and kinetics of the whole process. In the evaluation of the kinetic constants, the p K IH values of indole have been taken into account; in this way, the kinetic constant of formation of the dimer (kD ) results almost four order of magnitude larger than that of the trimer (kT ), suggesting a higher electrophilicity of the 3H-indolium cations with respect to the protonated dimer (which is an aliphatic ammonium salt). Further indole addition to the trimer, which is a protonated 2-alkyl- aniline, does not occur, since the anilinium ion is ineffective as an electrophile