The nature, structure, and composition of hydrocarbon products that form under oligomerization of ethylene on acidic zeolite H-ZSM-5 at room temperature (296 K) have been analyzed using a combination of in situ IR and C-13 NMR and ex situ C-13 NMR and GC-MS. It is shown that oligomers strongly retained by the zeolite framework represent a mixture of hydrocarbons which composition ranges from C-5 to ca C14+ carbon atoms. Adsorbed oligomers are bound to the oxygens of the zeolite framework to form alkoxy species. Alkoxy species represent main adsorption state of the oligomers, but they are in equilibrium with olefins and carbenium ions of the same composition, the concentration of both carbenium ions and olefins seems to be beyond the sensitivity of both NMR and IR. The reactivity of the adsorbed oligomers is defined by the reactivity of equilibrated carbenium ions which can easily interact with carbon monoxide and water to give a mixture of corresponding carboxylic acids. Oligomeric alkoxides with linear hydrocarbon fragments prevail in the mixture of the formed oligomers, The average length of the linear oligomer chain includes approximately seven carbon atoms. Prevalence of the linear hydrocarbon fragments over branched ones in adsorbed oligomers seems to be defined by both specific framework and dimensions of the channels of the zeolite H-ZSM-5.