We have fully characterized the three-dimensional morphology of thin films of mixtures of polystyrene and polybutadiene cast from a toluene solution, using nuclear reaction analysis, neutron reflectometry and transmission electron microscopy. Polystyrene-rich phases wet both the air and substrate interfaces and are separated by a polybutadiene-rich phase; these layers are very well defined and the interfaces between them are sharp (down to < 20 angstrom). Within the polybutadiene-rich central layer, lateral phase separation is also evident, with polystyrene-rich domains of oblate spheroidal shape. Under certain circumstances thin polystyrene-rich layers exist within the polybutadiene-rich phase. We discuss possible mechanisms for this unusual morphology in terms of surface effects on the mechanism of phase separation in the ternary polymer-solvent system from which the films are cast.