A detailed X-ray scattering study of the phase transitions in GdB(6) has been performed. For photon energies away from resonance, peaks are observed with two distinct wave vectors of q(1) (0 0 1/2) and q(2) = (1/2 1/2 0) at low temperatures. These are shown to arise from distortions of the lattice, although an earlier study had attributed them to competing magnetic-order parameters [J. Appl. Phys. 63 (1998) 3580]. With the photon energy tuned to the Gd L(2) edge, a third set of peaks with q(3) = (1/4 1/4 1/2) was discovered. This is shown to be the true magnetic-order parameter. Our results reveal a surprisingly rich interaction between the lattice and spin degrees of freedom in this compound. (C) 2003 Elsevier B.V. All rights reserved.