We plan to use the OFFSPEC SPIN-Echo reflectometer to study the mechanism of the 'peristaltic mode' in the spinodal dewetting of thin polymer films. In the peristaltic mode the displacements of the free surface and the fluid/fluid interface are in antiphase and the ratio of the amplitudes of displacements of the surface and the interface are predicted to be equal to the ratio of the surface tension to the interfacial tension. Two cases will be investigated where we have already a deep knowledge of the systems: 1) bilayer system where the thin bottom film is dewetting (the displacement of the polymer/polymer interface is the important one); 2) a tri-layer system: a thin polymer film between two thick polymer films (we expect the displacements of the two interfaces to be similar). We plan to monitor the early development of the unstable mode at interfaces by specular and off-specular