Reactions of orthophthalaldehyde (OPA) with amines are used in the determination of amino acids and in applications of OPA as a biocide. To contribute to the understanding of processes involved, the reactions of OPA with ammonia, which are conveniently slow, were studied. In a set of rapidly established equilibria, the 1,3-dihydroxyindole and the product of its dehydration are formed (Scheme 1). The individual equilibria were identified and equilibrium constants determined using DC polarography and UV spectra. The ring closure involves the carbinolamine; the imine formation is a side reaction. Both the ring formation and the dehydration of the carbinolamine are generally acid catalyzed. In the finally established overall equilibria between OPA and the isoindole derivative, the concentrations of intermediates are negligible. The same applies to the reaction of OPA with 2-aminoethanol, in which the initial formation of a carbinolamine and of an imine are too fast to be followed. Very slow reactions taking place during periods of hours or days, which probably result in the formation of dimeric species, have also been observed. This contribution demonstrates the advantages of combinations of polarographic and spectrophotometric techniques in the investigation of complex reactions of some organic compounds.