Gas chromatography/mass spectrometry (GC/MS) studies of model reactions with 2,4,4-trimethyl-1-pentene (TMP1) initiated by the system 2-isothiocyanato-2,4,4-trimethylpentane (H-TMP-NCS)/titanium tetrachloride (TiCl4) were made in the presence or in the absence of a proton scavenger 4-methyl-2,6-di-tert-butylpyridine (MDtBP). The results show the participation of two initiation modes, direct initiation and cocatalytic initiation, with H-TMP-NCS not being an initiator. The exact role of each mode can be separated. In the presence of MDtBP, direct metallation leading to H-TMP-Cl is evidenced upon quenching with MeOD. In particular, this experiment brings some precision about the complexity of the quench reaction. Dimerization of TMP1 leads to a mixture of four main isomers whose structures are specified by charge-exchange MS with CS2 gas and confirmed by a microhydrogenation reaction. This work demonstrates that H-TMP-NCS is not an initiator, and leads to the conclusion that when an isobutene polymerization is carried out in the presence of H-TMP-NCS, the functionalization is obtained by exchange reaction.