Siliceous FSM-16 possesses acid sites to catalyze but-l-ene isomerization to produce but-2-ene (cis/trans = 1.4-1.7) at 323 K and 2,6,6-trimethylbicyclo[3.1.1]hept-2-ene (alpha-pinene) isomerization at 303 K. Catalytic activity was dependent upon heat treatment and reached a maximum at 673 K. The maximum acid strength was invariably H-0 = -3.0 independent of the pretreatment temperatures. The acidity was much reduced by calcination at higher temperatures, but restored by water treatment at 353 K as long as the FSM-16 retained its structure.