The aim of this paper is to evaluate the effects of cocoa polyphenols and procyanidins with different degrees of polymerization that are encapsulated in liposome delivery systems on the inhibition of lipid oxidation at pH 3.0 and 5.0. In general, liposomes at pH 3.0 and 5.0 were physically stable in the presence of polyphenols and procyanidins with mean particle sizes of 56.56 ± 12.29 and 77.45 ± 8.67 nm and ζ-potentials of -33.50 ± 3.16 and -20.44 ± 1.98 mV at pH 3.0 and 5.0, respectively. At both pH 3.0 and pH 5.0, all the polyphenols and procyanidins inhibited lipid hydroperoxide and hexanal formation, and antioxidant activities increased with increasing polymer-chain sizes. The greater antioxidant activities of the isolated procyanidins were likely due to their increased metal-chelating capacities, as determined by ferric-reducing-ability (FRAP) assays, and their greater levels of partitioning into the lipids, as determined by their log Kow values and encapsulation efficiencies. The crude extract had the greatest antioxidant activity, which could be because other antioxidants were present, or combinations of the different polyphenols and procyanidins inhibited lipid oxidation synergistically.