Solid-state Si-29 and Mg-25 MAS NMR spectroscopy, in conjunction with thermal analysis and X-ray powder diffractometry, indicates that the thermal decomposition of talc is an essentially simple reaction in which the loss of structural water is concomitant with the atomic rearrangements involved in the formation of orthoenstatite with silica separation. The Mg-25 MAS NMR spectra of talc, enstatite and partially converted materials suggest no major differences in the octahedral cation environments of these phases, and provide no evidence for the incorporation of Mg into the amorphous silica phase. Neither the Si-29 nor the Mg-25 spectra indicate the formation of a reaction intermediate such as a pyribole, which, if formed, may however have characteristics which are too similar to the talc and pyroxene units from which it is composed to be distinguished by NMR.