Study of the reaction of trans-1,3-diphenyl-1-butene (D, the trans ethylenic dimer of styrene) with triflic acid in dichloromethane has been carried out using stopped-flow technique with UV-visible spectroscopic detection. Product characterizations and cation observations have been previously described, and the present work consists of a kinetic study of the reactions which can be considered as model reactions for the behavior of the polystyryl cations in the styrene cationic polymerization, after complete conversion of the monomer. The main observed cationic species were 1,3-diphenyl-1-butylium (D+) and oligomeric cations (D-n(+)); they absorbed at 340 nm and had a transient existence. Absorbance at 340 nm first increased very fast to reach a maximum value within a few seconds, after which the peak slowly decreased. Its persistence time was shortened when either acid concentration or temperature were increased or when the D concentration was decreased. From the analysis of the rate of appearance of the 340 nm peak, kinetics of protonation of D has been studied. Reaction was first order with respect to D and 1.26 order with respect to acid. The absorbance at 340 nm reached a maximum (ODmax), the value of which increased when acid and D initial concentrations were increased and when temperature was lowered. Concentration of cations remained always very low as compared to the initial concentrations of reagents. As for the polymerization of styrene initiated with triflic acid, the low amount of propagating cations indicates fast deprotonation either by beta-elimination or by indanic cyclization.