Commercial membranes suitable for solid alkaline membrane fuel cell (SAMFC) have not proved their competitiveness yet, due to poor ionic conductivity and water and fuel retention. Plasma polymers are however promising candidates to meet the SAMFC requirements. In this work, X‐ray reflectometry has been used to determine the thickness, density and water swelling of plasma polymerized films prepared from triallylamine, leading to a bilayer model for the studied thin films. Both film density and water swelling show a bimodal evolution as a function of the average plasma discharge power, which is directly related to the precursor fragmentation mechanism in the plasma phase and can be correlated with the film chemical composition.