Second order structural phase transitions in Alur 6(ClO 4) 3 and Gaur 6(ClO 4) 3 with T c ∼ 300 K are studied by means of ESR on single crystals doped with the analogous Cr(III) compound. The transitions are antiferrodistortive and of the displacive type, the displacements resulting from the condensation of a X 2 mode (k = (0 1 2 1 2 )) of the ClO 4 ions. The ESR parameters have the same temperature dependence as the order parameters and can be described by D and E∼φ∼. The space group describing the structure changes from S 6 2 to S 2 1, and the number of domains is multiplied by three. Above 300 K the crystals already consist of two domains, resulting from a ferrodistortive phase transition D 3 d 6 → S 6 2. The actual transition temperature of the latter phase transition lies at some temperature above the decomposition temperature of the crystals.