We have studied the thickness dependence of the spinodal decomposition in thin films of a binary polymeric mixture. Nuclear reaction analysis (NRA) and time-of-flight forward recoil spectrometry (TOF-FRES) have been used to monitor the phase separation in thin films of poly(ethylenepropylene) (PEP) and perdeuterated poly(ethylenepropylene) (dPEP) after a quench into the two-phase region. The composition profiles are modeled by two distinct spinodal waves originating from the two surfaces of the films, and interference effects are observed as the film thickness is varied. Below a critical film thickness the coarsening of the phase structure is found to be severely altered with respect to the thick film behavior. Cell-dynamical simulations have been performed which closely resemble the experimental findings.