Flash vacuum pyrolysis (FVP) of 4-acetyltetrazolo[1,5-a]pyridine 5 at 400 °C provides 3-methyl isoxazolo[3,4-b]pyridine 6 whose structure was confirmed by X-ray crystallography. At higher pyrolysis temperatures, the unstable heteroindoxyl 8 was obtained, which exists as the keto form (1,2-dihydropyrrolo[2,3-b]pyridin-3-one) 8K in CDCl(3) solution and the enol tautomer (3-hydroxypyrrolo[2,3-b]pyridine) 8E in DMSO. The heteroindoxyl 8 oxidatively dimerises to the heteroindigotin 9, undergoes condensation reactions at the 2-position and reacts with methoxymethylene Meldrum's acid at the 1-position. FVP of the corresponding acetyltetrazolo[1,5-a]quinoline 19 was much more complex, with 2-(cyanophenyl)acetonitrile 30 (rather than a heteroindoxyl) the major product at 750 °C. FVP of 3-acetyl-4-azidoquinoline 24 at 400 °C gave 3-methylisoxazolo[4,3-c]quinoline 33, but rearrangement to the heteroindoxyl was not observed at higher temperatures.