Imogolite, protoimogolite and two synthetic allophanes (A and B) with very different Si/Al ratios have been pyrolysed over a range of temperatures, and the reactions followed by high-resolution solid-state 27AI and 29Si nuclear magnetic resonance (NMR) spectroscopy. The Si(OAloct)3OH groups of protoimogolite are destroyed at relatively low (∼ 200°C temperatures, whereas the same structures in imogolite are stable to ∼ 300°C. During pyrolysis of both materials there is little change in the observed coordination of AI at temperatures of < 500°C but at 500°C or higher, ∼25% of the NMR-visible Al is converted to tetrahedral coordination. The mechanism of decomposition of both protoimogolite and imogolite is shown to involve the formation of highly branched Si-O-Si chains. As far as can be discerned by 27Al and 29Si NMR, allophanes A and B both appear to produce products similar to protoimogolite on pyrolysis.