The thicknesses of polyelectrolyte multilayer thin films (PEMs) are reported as a function of deposition temperature and ionic strength. PEMs fabricated at high salt and elevated temperatures are significantly thicker than similar films deposited at room temperature. These trends in the multilayer films are not observed in single-layer films of PDDA directly deposited onto Si substrates. Based on the results of this study, a new mechanism is proposed to explain the dependence of PEM thickness on temperature. It is believed that elevated temperature swells the existing film and allows for increased trapping of solution-phase polymer within the already-existing layers. The results and interpretation have bearing on the present understanding of PEM film deposition and also on the ability to control interpenetration between individual polymer layers.