X-ray and neutron scattering have been used to study the temperature, wavevector, and frequency dependences of short-range correlations developing in various crystals ofrb1-x(nh4)xh2P04,andinRb0.35(ND4)0.65D2PO4,adeuterated homolog, as they freeze into a glass like state. Diffuse incommensurate peaks grow on cooling for wavectors along the  and  directions for values of x in the glass-forming range. This diffuse scattering, including the x-dependence of its wavevector, is qualitatively well explained by a model Hamiltonian that takes into account the crystal structure and the competing ferroelectric (FE) and antiferroelectric (AFE) interactions, without adjustable parameter. Observations close to the FE boundary indicate that the incommensurate correlations are very much quenched by FE ordering. Close to the AFE boundary, diffuse peaks also appear at the AFE wavevectors. Neutron scattering reveals a dynamical origin of the incommensurate correlations, with a temperature-dependence of their characteristic frequency that so far seems in agreement with the dynamics observed in macroscopic (q = 0) measurements. © 1988, Taylor & Francis Group, LLC. All rights reserved.