In this work, we report on the stability of oxygen-rich plasma-polymerized (pp) films in an aqueous environment. The pp films were deposited via atmospheric-pressure plasma jet treatment of polymerizable organic liquids. The monomers used for the plasma-assisted polymerization were tetrahydrofurfuryl methacrylate, 1,2,4-trivinylcyclohexane, and mixtures thereof. The pp films were deposited at different plasma input powers ranging from 3 to 7 W. The stability of the obtained pp films was studied upon long-time storage in pure water and in buffer solutions of pHs 4, 7, and 10. After 24 h of storage of the pp films in de-ionized water, all of the studied pp films experienced thickness losses along with the formation of various ringlike structures at their surface, whereas Fourier transformed infrared (FT-IR) analysis showed no changes in their chemical composition. The pp films stored in pH 10 were completely delaminated from the substrate surface, while the pp films stored for 24 h in pH 4 showed swelling behavior, partial delamination, and the formation of wrinkles at the coatings' surface. The pp films stored for 24 h in pH 7 experienced minor thickness losses and formation of wrinkles at their surface. FT-IR analysis of the pp films stored in buffer solutions of pH 4 and pH 7 showed a decrease of C=O and an increase of O-H stretching signals in all of the cases. The observed chemical changes corresponded to the hydrolysis of esters presented in the pp films' structure.