Results of a series of synchrotron radiation scattering studies of a Pb(MgNb)O (PMN) single crystal are presented. The temperature dependence of the 1:1 chemical short-range order (SRO) peaks, e.g. 1/2(115), was studied in detail from 40 to 473 K. At low temperatures, the temperature dependence is very strong, but as the temperature is raised above room temperature, the dependence becomes very weak. Moreover, a secondary peak becomes evident below 100 K. It is suggested here that this change in behavior is due to an anti-phase tilting of the oxygen octahedra coupled with some ordering of the Pb displacements. The temperature dependence of the H2(hk2l) positions, e.g. 1/2(015) and 1/2(017), were carefully studied through the transition down to 40 K, as well. Peaks at these positions appear at low temperatures due to the in-phase tilting of the BO octahedra, but its long-range order is never established. These results imply that both ferroelectric and anti-ferroelectric fluctuations are present in the structure and are evidence of competing interactions in PMN. Along with the quenched disorder due to the B-site randomness, these interactions induce orientational frustration of electric dipoles (Pb-O) in the structure and lead to the observed dielectric behavior. Thus, PMN possesses the underlying physical characteristics of a dipole glass system. © 1999 Elsevier Science Ltd. All rights reserved.