An organic-inorganic perovskite compound [(CH3)4P][Cd(SCN)3] (1) and its fluorine-substituted product [(CH3)3PCH2F][Cd(SCN)3] (2) exhibit ferroelastic phase transitions above room temperature. The very close van der Waals radii of H and F atoms ensure isomorphism of the crystal structures. However, the higher phase transition temperature, stronger ferroelastic spontaneous strain value and dielectric properties of 2 can possibly be explained by differences in the electronegativity between F and H atoms.