ABSTRACTThe thickness‐dependent water vapor swelling of molecular layer‐by‐layer polyamide films is studied via specular X‐ray reflectivity. The maximum swelling ratio of these ultrathin films scale inversely with thickness but more importantly show a dual‐mode sorption behavior characterized by Langmuir‐like sorption at low relative humidity and network swelling at high relative humidity. The thickness‐dependent network parameters are extracted using a proposed model that builds on Painter‐Shenoy network swelling model while taking into account the glass‐like characteristic below a critical swelling ratio, which also scales inversely with thickness. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 412–417