Spin- coating is a very widely used technique for making uniform thin polymer. films. For example, the active layers in most experimental semiconducting polymer- based devices, such as light- emitting diodes and photovoltaics, are made this way. The efficiency of such devices can be improved by using blends of polymers; these phase separate during the spin- coating process, creating the complex morphology that leads to performance improvements. We have used time- resolved small- angle light scattering and light reflectivity during the spin- coating process to study the development of structure directly. Our results provide evidence that a blend of two polymers. first undergoes vertical strati. cation; the interface between the stratified layers then becomes unstable, leading to the final phase- separated thin film. This has given us the basis for establishing a full mechanistic understanding of the development of morphology in thin mixed polymer films, allowing a route to the rational design of processing conditions so as to achieve desirable morphologies by self- assembly.