We studied the phase separation of a polymer solution in thin slabs of thickness of micrometers. The material used was an asymmetrical binary system that consisted of 6 wt% semi-crystalline poly(ethylene butene) (or cEB) and 94 wt% of diisodecyl adipate (or DIDA) solvent. The de-mixing mechanism in the confined geometry is very different from that in bulk. Phase separation in bulk usually starts with formation of the solvent holes, and polymer-rich phase becomes a micro-reticulated network. While, in thin films the phase separation starts with formation of the polymer-rich droplets, that behave like Brownian particles. We have observed that there exists a critical thickness L-c below which the size of the droplets decreases drastically. Above the critical thickness, we have observed a phase morphological transformation from droplets to chains, branches, and then to networks, which follows a very similar pattern of a percolation process. There is correspondingly a second critical thickness L-c(*) (> L-c) below which no bi-continuous phase separation takes place. (C) 2604 Elsevier Ltd. All rights reserved.