Effects of disordering of Se and Nb in the relaxor ferroelectrics Pb(ScNbO (PSN) were investigated by means of measurements of x-ray scattering and dielectrcity. Results on the diffuse scattering from ordered PSN indicate lattice instabilities at the F and M points near 353 K, and these were expected to cause the formation of ferroelectric (FE) and antiferroelectric (AF) phases. The freezing behaviours are slightly diffuse because of the partitioning of the thermal energy to the two modes, but yet of the type characteristic of normal soft-phonon modes. The results of the dielectric constant measurement under an alternating-current field indicated a highly polydispersive dispersion and a significant temperature dependence of the relaxation frequency. These unusual results imply a competing interaction between the two lattice instabilities. The x-ray work on disordered PSN shows 'diffuse' transition behaviours at the two points. It is suggested that this feature originates from lattice relaxation through the softening of the two modes. The distortions are self-accommodated in atomic displacements at the M point, and analysis of the intensities suggests that the displacement pattern has the form of an AF distortion. During the FE freezing, the coexisting FE and AF regions may give rise to a dipole-glass state, and this is thought to cause relaxor behaviours through frustration between the two states.