Ultra-thin layers (h = 50-150 nm) of PVC, functionalised in solution by thiophenol (t-PVC) and 4-aminothiophenol (am-PVC), were prepared by spin-coating an silicon wafers. The glass transition temperature of these thin films was investigated by kinetic scan ellipsometry.. The Tg of the am-PVC and t-PVC modified PVC is found to increase more strongly with the degree of modification in thin layer geometry than in the hulk Also, Tg(h) of t-PVC exhibits a larger increase in the confined state as compared to am-PVC. This result is unexpected since no specific interactions are assumed between thiophenyl group and the surface indicating that other driving forces for the alteration of Tg(h) should be taken into account. In a second step, the supported PVC thin films are chemically cross-linked in solution by terephthaloyl chloride. The Tg(h) of the polymer layer increases with the cross-linking time more severely at a higher degree of modification but independently of the thickness of the layer indicating that the diffusion process is not altered in thin layers. Above Tg, the thermal expansion coefficient cu of the thin films is not modified after cross-linking, whereas, it was found to decrease strongly between 4 and 8% of modification in the uncured state.