Ferroelectric-ferroelastic rare earth (RE) molybdates RE2(MoO4)(3) have been prepared by the conventional solid-state synthesis and X-ray powder diffraction has been collected using two diffractometers PANalytical X'Pert Pro at room temperature. The crystal structures have been calculated by Rietveld refinement, using a new symmetry modes procedure in order to evaluate the correlation between the distortions, with respect to the paraelectric-paraelastic structure, with the ionic radii. Although the coordinates of the oxygen atoms are not very precise and accurate, we have observed different behavior of the ferroelastic-ferroelectric order parameter.