Using nuclear reaction analysis composition-depth profiling, we investigate the influence of symmetric/asymmetric confining walls on the equilibrium configuration of thin films of phase-separated polymer blends. Depth profiles of samples annealed under symmetric boundary conditions show a laterally averaged concentration, while samples confined by nonsymmetric walls show (as in earlier studies) clear separation into two thin layers of coexisting phases. This suggests that for phase separation under symmetric boundary conditions the interface between the two phases is orthogonal to the sample plane, in line with recent theoretical discussion.