Ti-substituted HMS catalyst has been prepared and applied as a bifunctional catalyst to the two-step conversion of alpha-pinene via alpha-pinene oxide to campholenic aldehyde using TBHP as an oxidant. The reaction mainly proceeds by a radical mechanism, leading to the formation of allylic oxidation products like verbenol and verbenone. To suppress the formation of byproducts, other oxygen sources except for the oxidant are removed from the reaction system. In the presence of water, the hydrophilic character of Ti-HMS can induce the preferential accessibility of water, resulting in the increase of intraporous water concentration and its subsequent blocking of catalytic sites. Also, in the presence of oxygen, the propagation of radicals is promoted and this leads to the formation of other side-products. After removing water and molecular oxygen, it is possible to synthesize campholenic aldehyde in one-pot over Ti-HMS catalyst. Therefore, the removal of other oxygen sources that may lead to the allylic oxidation is essential for the one-pot conversion of alpha-pinene to campholenic aldehyde. (C) 2003 Elsevier Science B.V. All rights reserved.