Thermal effects in organo-metal halide perovskites are studied by ab initio molecular dynamics (MD) simulations performed at effective temperatures of 293 and 383 K and by X-ray photoelectron spectroscopy (XPS). We find that the cause of thermal instability in this class of perovskites is the rotation of the methylammonium (MA) groups that destroy the rigid lattice of pure compounds (MAPbI(3) and MAPbBr(3)). When the Pb-I lattice is initially distorted by partial replacement of the I with CI or Br, this not only prevents formation of PbI2 seeds but also improves lattice flexibility and stability against the temperature-induced motion and rotation of MA groups.