X-ray scattering techniques have been used to study the diffuse scattering from a single crystal of Rb1-x(ND4)xD2PO4 with x=0.65. This system has a structural glass phase at low temperatures resulting from the competing ferroelectric interactions of RbD2PO4 and antiferroelectric interactions of (ND4)D2PO4. The diffuse scattering shows a broad peak with a maximum occurring at a wavevector of about 0.3 a*, and the intensity of these peaks is surprisingly different for wavevectors q on opposite sides of the Bragg reflections. A model of the D bonding is developed which suggests that the diffuse scattering arises from the interaction between ferroelectric displacements along c, ferroelectric displacements along b, and transverse acoustic modes polarized along b and c. The model accounts for the incommensurate wavevector and, qualitatively, for the intensity of the diffuse scattering around different Bragg reflections. The temperature dependence of the scattering is also measured. © 1986 Springer-Verlag.