The Lewis acid initiated cationic homopolymerization of Norway fish oil ethyl ester (NFO) or the corresponding conjugated fish oil (CFO) and their copolymerization with various alkene comonomers is investigated. Among the Lewis acids employed, boron trifluoride diethyl etherate (BFE, BF3 . OEt2) is found to be the most effective initiator for cationic polymerization of the NFO and CFO systems. The BFE-initiated homopolymerization of NFO generally results in low molecular weight viscous oils, while that of CFO leads to a solid elastic gel with a gel time of more than 72 h at room temperature. Copolymerization of the NFO or CFO with some alkene comonomers significantly facilitates gelation. The gel times are largely dependent upon the stoichiometry, the type of fish oil, and the alkene comonomer. After postcuring at elevated temperatures, the cationic copolymerization affords polymers ranging from soft rubbery materials to rigid plastics. These NFO and CFO polymers are composed of highly crosslinked materials and a certain amount of free oils and are found to be fully cured thermosets. Generally, CFO polymers appear to be harder than the corresponding NFO polymers. However, the thermal properties of the bulk polymers are similar to each other, and their insoluble extracts exhibit much higher thermal stability than the bulk thermosets.