Polymeric films thinner than a 100 nm exhibits a variation of the glass transition temperature of a few tens of degrees and can brake up into discrete islands when heated near their glass transition temperature. Results from neutron reflectivity on polystyrene and PMMA have shown that according to the molar mass, volume fractions of solvent up to 10% were trapped in the bulk of the film and at the polymer substrate interface. The impact of this residual solvent on the dewetting process and the glass transition shifts is still unclear. We intend in this proposal to clarify the conditions of the existence of residual solvent in spin coated ultra thin films in order to better understand its implication in the ultra thin properties and probe the dynamics of ultra thin films PMMA films by following the diffusion of either a good solvent or a theta solvent.