The growth, structure, and dynamics of RbH2PO4-NH4H2PO4 mixed crystals are reviewed. In the intermediate concentration range, the built-in frustration of these materials prevents the ordinary ferroelectric or antiferroelectric transitions, whereby the disordered acid-proton sublattice freezes into a glass. This has been observed with a number of experimental techniques, including dielectric dispersion, optical birefringence, X-ray diffraction and scattering, Raman and Brillouin scattering, NMR, and more recently, specific-heat and neutron scattering. The variety of techniques applicable to this system has allowed the observation of polarization freezing over about 17 orders of magnitude in frequency. This freezing is well described phenomenologically by the Vogel-Fulcher law, while other laws proposed for spin- glasses are less successful. Recent hypersonic-relaxation measurements point to the existence of a frozen polarization component at temperatures already much above Vogel-Fulcher freezing, while neutron diffused scattering reveals that the tendency to form superstructures is not static. © 1985 The Japan Society of Applied Physics.