The carbonaceous deposits formed on LaHY-FAU used in a semi-batch reactor at 353 K to alkylate isobutane with 1-butene have been examined by in situ W-VIS, FT-IR, and C-13 CP/MAS NMR spectroscopies. Only UV-VIS studies evidence unsaturated carbenium ions immediately on adsorption of 1-butene and or 1-butene/isobutane mixture. These ions are part of high molecular weight oligomers (low-temperature carbonaceous deposits) which remain attached to the active site, effecting catalyst deactivation by poisoning. Unsaturated carbenium ion formation is favored by higher reaction temperature (up to 523 K), longer contact times (lower LSHV) and increased Lewis acidity. The presence of isobutane greatly reduces oligomer formation. Thermal treatment of the used catalyst under vacuum causes the low-temperature deposits to desorb from the active sire and conversion to aromatics (which remain up to 823 K). The acidic OH groups of the active sites are to a large part regenerated by this treatment yet the initial catalytic activity cannot be fully restored.