Various reactions of 1,3,4,7-tetramethylisoindole are described. The tautomerism with the corresponding 1H-iso-indole in various solvents is examined. Autoxidation leads to two compounds which are formulated as 1-hydroxy-and 1-hydroperoxy-1H-isoindole derivatives. The addition of dienophiles is complex : maleic anhydride gives the Diels–Alder adduct (IX) which subsequently isomerises (on melting, or in solution) to give (X). Dimethyl acetylenedicarboxylate gives both 1 : 1 and 1 : 2 adducts : structures are proposed for these compounds. When heated with a metal powder or a metal salt 1,3,4,7-tetramethylisoindole gives the metallo-octamethyltetrabenzporphyrins.