A single crystal of Ag-36.7 at.% Al aged within the hexagonal close-packed (h.c.p.) delta-phase region of the phase diagram at 366 K was investigated by diffuse X-ray scattering. Diffuse scattering maxima were found at 2k(F) positions where k(F) is the Fermi wave number along (110]. The contributions from short-range order scattering, linear size-effect scattering, Huang scattering and thermal diffuse scattering were separated by least-squares fitting of the corresponding Fourier series. In crystals modeled on the basis of 40 Warren-Cowley short-range order parameters, nearest-neighbor configurations are found which are characteristic of a superstructure introduced by Neumann as a possible ground-state structure of h.c.p. Ag-Al. Effective pair interaction parameters of the first 12 shells determined by the inverse Monte Carlo method were used in Monte Carlo simulations. For the Neumann structure, an order-disorder transition temperature of similar to100 K is estimated. (C) 2003 Elsevier Ltd. All rights reserved.