The water vapor swelling behavior of ultrathin crosslinked aromatic polyamide (PA) films, synthesized via molecular layer-by-layer deposition, is characterized via specular X-ray reflectivity. The results show that these films expand only along the thickness direction and their maximum swelling ratio can be controlled by the specific diamine monomer chemistry. By measuring the swelling ratio as a function of water activity, the Flory interaction parameter and monomers between crosslinks of these PAs are characterized using the Flory-Rehner and the Painter-Shenoy thermodynamic swelling models. Comparing the monomers between crosslink calculated from the swelling theories with values obtain from chemical analysis indicate that the PS model gives a more accurate description of the PA networks. This suggests that these low functionality but highly crosslinked networks swell via a combination of chain expansion and topological rearrangement. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1647-1655