Abstract The i.r., visible and u.v. spectra, and the room temperature magnetic susceptibilities of the hydrated and anhydrous chromates (VI) and of the chromites of magnesium, lanthanum, neodymium and samarium have been measured. The course of dehydration of the hydrated chromates and the course of decomposition of the anhydrous chromates have been investigated by differential thermal analysis and by thermogravimetric analysis. The stoicheiometry of the decomposition of the chromates has been investigated and, in a search for intermediate products, i.r. spectra, X-ray powder diffraction patterns and magnetic susceptibilities have been measured on series of partially-decomposed samples. The kinetics of the isothermal decompositions were studied thermogravimetrically; it was shown that the results were well represented by some of the standard kinetic equations and activation energies were derived. The relation of the activation energies to the spectroscopically-determined charge-transfer energies are interpreted as showing that the activation process for the thermal decomposition is a one-electron transfer from oxygen to chromium. The propagation of the activation process is discussed in relation to the crystal structure and the i.r. spectra of the chromates.