Solvent mass uptake and the swelling of ultrathin polyelectrolyte films (within thickness range 157-375 Å) in presence of water vapor have been performed to study the effect of long range Coulombic interaction between charged segments of a polyelectrolyte chain on its swelling dynamics. Films were prepared from aqueous solution of poly(sodium-4-styrenesulfonate)(NaPSS) on hydrophilic silicon substrate using spin coating method. Diffusion coefficient of water has been obtained from the solvent mass uptake study using gravimetric measurement and was found to increase with the increase of film thickness. Swelling dynamics of the polymer films in presence of water vapour, on the other hand, have been studied in-situ using x-ray reflectivity technique. Diffusion coefficient of the NaPSS chains has been found to be of the order of 10-18cm2/s and was independent of the film thickness. Whereas, the fraction of charged monomer which determines the strength of the repulsive interaction between the polymer segments, shows confinement effect and increases linearly with the film thickness.