Plasma-oxidized polyethylene (0-PE) was derivatized with various chemicals and the advancing and receding contact angles with water, formamide, and tricresylphosphate were measured. The contact angles were used to evaluate the surface free energy (SFE) components in terms of the Lifshitz-van der Waals-acid base (LW-AB) model. The derivatization was directed to couple alkyl chains to carbonyl, hydroxyl, and carboxyl groups generated on the surface of O-PE. Carbonyl groups were treated with hydrazine and then reacted with dodecanoyl chloride. Hydroxyl groups were first reacted with reactive dicarboxylic acid dichlorides or cyanuric chloride, respectively, and then with alkylamines because the direct bonding of long-chain alkyl carboxylic acid chlorides proceeds slowly and is, therefore, completed by surface restructuring. Carboxyl groups were chlorinated and in a second step reacted with alkylamines. The bonding of C-12-alkyl chains can be sensitively detected by the LW part of the SFE. Concerning the receding contact angles, it is pointed out that the carbonyl groups are present on the surface in a higher concentration than hydroxyl and carboxyl groups. The combination of contact angle goniometry (CAG) and chemical derivatization reactions supplies semiquantitative information about functional groups in the uppermost surface layer. In this way, a major shortcoming of CAG can be overcome.