X-ray measurements on single-crystals of Rb1-x(NH4)xH2PO4 trace the development of a glasslike state. The absence of macroscopic crystal-symmetry change during freezing is confirmed, while diffuse scattering reveals the growth of very-short-range order. The latter coincides with a compressibility anomaly seen in Brillouin scattering and with dielectric effects suggesting freezing. The system is in many ways analogous to spin-glasses. © 1984 The American Physical Society.