27AI and 2~Si magic-angle spinning(MAS) nuclear magnetic resonance(NMR) and complementary X-ray diffraction (XRD) studies of carbothermal formation of sialons from kaolinite and halloysite confirm that the reaction involves the initial formation of mullite (3AI203.2SIO2) and amorphous silica. In the presence of carbon, Si-N bonds are formed at ~1200~ giving a continuum of silicon oxynitride compositions which become progressively more N-rich. These do not become sufficiently ordered to be detected by XRD until later in the reaction, when crystalline silicon oxynitride, possibly containing a little AI (O'-sialon) and x-phase sialon are formed, followed by lY-sialon. The O'-and x-phase sialons are transitory, but the I~'-sialon persists throughout the reaction. Si-O bonds survive the destruction of the mullite and persist throughout the reaction, especially with kaolinite starting material. The 2~Si MAS NMR results indicate that Si-C bonds are formed later in the reaction than previously suggested, the SiC phase behaving more like a secondary product than a transitory intermediate. AI-N bonds are not detectable by 27AI MAS N M R until very late in the reaction (after 8 h firing at 1 400 ~ and coincide with the appearance of the secondary product AIN. The implications for the carbothermal reaction sequence in kaolinite and halloysite are discussed.