We consider a 3D Ising lattice which is partially disordered, i.e. in which the spin at site (i,j,k) has a probability n_(ijk) of pointing in the in the ‘up’ direction. The spins are subjected to bilinear interactions which are such that the spin average value is constant within each xy plane, and varies periodically in the z direction. This variation has a sinusoidal character right below a critical temperature above which the structure is totally disordered, and is of the square-wave type at T = 0. In this paper the spatial variation of the disorder is studied by looking at the harmonic components of each modulation. The lattice is treated as a 1D chain to each site of which the average value p_k of the spins of an xy plane is associated. We work at temperatures slightly lower than the critical temperature of the modulation, and carry out an expansion up to second order (in one case we go up to third order) in the p_k's. We discuss how the results can be applied to the study of transitions between modulated structures, and in particular between structures with the same wavelength and different symmetry.