The structural evolution of PbSc0.5Nb0.5O3 (PSN) under pressure has been studied by in situ powder neutron diffraction. Rietveld refinements to the data show that the continuous phase transition detected by x-ray diffraction at pc = 4.1 GPa (Maier et al 2010 Phys. Rev. B 81 174116) is associated with long-range ordering of antiphase octahedral tilts and local ordering of ferroic Pb displacements. Similar to PbSc0.5Ta0.5O3 (PST) (Maier et al 2010 Acta Crystallogr. 66 280–91), antiphase octahedral tilting even exists below the critical pressure in a regime in which the structure retains a cubic metric. In contrast to PST, in which the Pb atomic displacement parameters (DPs) form ellipsoids elongated along the cubic 111 directions, the atomic DPs of Pb in PSN form flattened discs parallel to the cubic {111}-planes. This indicates that in PST the Pb displacements are along the cubic 111 directions, whereas in PSN the local Pb displacements are randomly distributed along the cubic 110 directions. The latter can be explained by the abundance of underbonded oxygen atoms associated with the chemical B-site disorder.