C-13 NMR study of "conjunct polymerisation" of 1-pentene in 95% sulfuric acid demonstrated that this is a carbenium ion reaction, where the esters are the precursors of carbenium ions. In the first step of the reaction a complicated mixture of primary and secondary mono- and dipentyl esters is formed. In an excess of the acid the esters are decomposed yielding aliphatic carbenium ions, which are then involved in secondary reactions resulting in the final products. Similar to heterogeneous catalysis on zeolites, the steady state concentration of the aliphatic carbenium ions during the reaction is, however, too low for their direct detection by NMR.