A mechanism of the strain transition in M3H(XO4)(2), especially in Rb3H(SeO4)(2), involved in the ferroelastic-superprotonic phase transition is proposed where a coupling between strain and mobile protons plays a crucial role. The two dimensional Kagome lattice on the ab-plane is extracted for proton's pathway, and the pair approximation of the cluster variation method and path probability method is applied to evaluate thermodynamics and proton dynamics: the first-order long-range strain transition and the strain-involved dc-conductivity are qualitatively discussed. Both behaviors suggest the present proposed mechanism of the ferroelatic-superprotonic phase transition. (c) 2007 Elsevier B.V. All rights reserved.