Formation of the spirocyclohexadienyl dimers (III) from diazonium salts (I) by reaction with sodium iodide in acetone supports a radical mechanism for the interaction of iodide ions with diazonium compounds; pyrolysis of (III) give N-alkyl-phenanthridones by migration of nitrogen and not of aryl in the spirodienyl radicals (II), but pyrolytic decomposition of (IIIa) in the presence of iodine gives the dienone (IX) in high yield.