The (NH4)(2)CuCl4 center dot 2H(2)O crystal exhibits anomalous thermal, piezoelectric, dielectric and elastic properties in the vicinity of the ferroelastoelectric phase transition at T-C = 200 K. Macroscopic order parameter is the h(36) component of the spontaneous piezoelectric tensor connecting polarisation P-3 and strain eta(6). The temperature change in h(36) component was proportional to (T-C - T)(alpha), where alpha = 0.58 +/- 0.05. Close to T-C changes in dielectric permittivity were negligible. High value of ac conductivity in the high-temperature phase results from protons jumping between disordered ions NH4+ and between molecules of crystallisation water. Only longitudinal ultrasonic waves exhibited an abrupt change upon the phase transition. The relaxation time of the order parameter was determined from anomalous changes in attenuation of the longitudinal waves. (C) 2014 Elsevier B.V. All rights reserved.