The property of acidic sites on MoO 3 -ZrO 2 was studied for n-heptane isomerization. A 2,6-lutidine IR study showed that the introduction of MoO 3 on ZrO 2 partially eliminated the absorbance band at 1605cm −1 ascribed to Lewis acid sites corresponding to the presence of the monoclinic phase of ZrO 2 and developed several Brönsted and Lewis acid sites with different acidic strengths. MoO 3 -ZrO 2 possesses a large number of relatively weak Lewis and Brönsted acid sites as well as strong acid sites. The active protonic acid sites in n-heptane isomerization were formed from molecular hydrogen through a spillover mechanism with the involvement of doublet bands at 1595 and 1580cm −1 ascribed to the Lewis acid sites corresponding to the presence of the tetragonal phase of ZrO 2 . No catalytic activity of MoO 3 -ZrO 2 for n-heptane isomerization was observed in the absence of the doublet bands at 1595 and 1580cm −1 and hydrogen in the gas phase.