Adhesion of a cathodically electrodeposited paint (E-coat) to aluminum alloys, Alclad 2024-T3, AA 2024-T3 and AA 7075-T6, was investigated to examine the influence of interfacial tension at the paint/metal interface. The surface energy of an aluminum plate was modified by depositing a plasma polymer of a mixture of trimethyl silane (TMS) and one of three diatomic gases (O-2, N-2, and H-2) by cathodic plasma polymerization. The contact angle (theta) of water on a modified surface changes as a function of the mole fraction of the diatomic gas. The plot of cos theta(PP) Of a plasma polymer as a function of the mole fraction of the gas crosses the plot of cos theta(EC) of the E-coat. The difference, Delta Cos theta = cos theta(PP) - cos theta(EC), is a parameter which indicates the level of interfacial tension at the paint/metal interface, Delta Cos theta = 0 represents the minimum interfacial tension. The adhesion of a cured E-coat on a panel was evaluated by the N-methyl pyrrolidinone (NMP) paint delamination time test. The maximum peak of adhesion test values plotted as a function of Delta Cos theta occurred around the zero point, Delta Cos theta = 0, indicating that maximum adhesion is obtained with minimum interfacial tension. Mixtures of TMS and N-2 On all three aluminum alloys studied consistently displayed longer delamination times in the NMP test than mixtures of TMS and O-2 or H-2.