A critical review of former investigations on the cationically induced oligomerization of N-vinylformamide points out that some of the results are not in agreement with the model of an cationic propagation mechanism. Special attention in the discussion of the results is given on the chain structure of the oligo(N-vinylformamide)s. In comparison with N-vinylformamide, the ability of different N-vinylamides to undergo a cationically induced polymerization to oligomeric products was investigated. Only the secondary N-vinylamides, N-vinylformamide (VFA) or N-vinylacetamide (VAcA), readily form higher oligomers with various cationic initiators. N-Methyl-N-vinylformamide and the cyclic N-vinylpyrrolidone (VP) dimerize with acidic catalysts, whereas N-methyl-N-vinylacetamide gives a mixture of low molecular products. The N-alkyl-substituted N-vinylamides are not able to undergo a oligomerization to higher oligomers (M-n > 200) under the conditions investigated. The cationic polymerization of N-deutero-N-vinylformamide (VFAD) and the H-2 NMR analysis of the obtained oligomers show that the nitrogen-bonded hydrogen (deuterium) is strongly involved in the proton transfer as well as the propagation reaction. From these results, we propose an alternative oligomerization mechanism for the oligomerization of secondary N-vinylamides. A nonionic pericyclic transition state is potentially suggested involving a N-formylimine end group and the monomer rather than a cationically active chain.