Abstract A group theoretical analysis of the second-order structural phase transition in (CH 3NH 3) 2MnCL 4 at 394°K and of similar transitions in ethyl and propyl compounds ( D 17 4 h → D 18 2 h ) was performed. The soft mode transforms according to the τ x 5-irreducible representation at the X-point of the Brillouin-zone boundary and its eigenvector is discussed. The transition is of the order-disorder type and is caused by a slowing down of the hindered rotation of NH 3-groups. Knowing the symmetry of the order parameter, a thermodynamic potential expansion was constructed and expected anomalies in material constants around the transition temperature are briefly discussed. The high temperature phase transitions in analogous copper compounds are explained as a sequence D 17 4 h → D 18 2 h → D 15 2 h . The second of these phase transitions is driven by a soft mode transforming as the τ Y 7-representation at the D 18 2 h Brillouin zone boundary.