The composition of acid soluble oil (ASO) formed during isobutane alkylation with alkene in the presence of trifluoro methane sulfonic acid (TfOH) was studied. Based on the data received, ASO was assumed to be a complicated mixture of unsaturated hydrocarbons with conjugated double bonds. Its main fragment is the substituted five-member cycle of the formula C20H36. ASOs formed (1) by interaction of 2-butene with TfOH; (2) during alkylation in the presence of liquid TfOH; or (3) by degradation of 2,2,4-trimethyl pentane, are practically identical and independent of preparation method. NMR and UV spectroscopy were used to study the interaction of ASO with TfOH. It was demonstrated that in the acid and hydrocarbon phases, a rather strong complex is formed with the composition ASO x 2 TfOH. Such an interaction of ASO with TfOH explains the reason for the observed poisoning effect of ASO in the alkylation reaction.