Mass spectrometry and thermogravimetric analysis are used to explore the thermal decomposition of carbon precursors (primarily the tripropylammonium cations) occluded within AlPO-5 (API) crystals prepared in various media (in the presence or absence of F ions, Si substations of P), with the aim to fabricate high-density 0.4-nm single-walled carbon nanotubes (SWNTs). It has been found that the tripropylammonium precursors exist in the as-synthesized crystals in three different forms: tripropylammonium fluoride, hydroxide, and tripropylammonium cation compensating for the negative charge of the framework. The latter is bonded to the framework by strong chemical interaction and its decomposition takes place by a series of β-elimination reactions to give propylene and ammonia, with the stepwise formation of dipropylammonium and n-propylammonium cations. The 0.4-nm SWNTs filling density was found to be higher than that resulting from the carbon precursor of tripropylammonium fluoride and hydroxide, because of the strong adsorption force of the channel walls to pyrolysate, as evidenced by the clear and strong radial breathing modes in Raman spectra. © 2006 American Chemical Society.