The isomerization of alpha-pinene in hot compressed [GRAPHIC] water (HCW) at 250 degrees C and 7 MPa using TiO2 and W0(x)/TiO2 catalysts was investigated. In HCW, alpha-pinene mainly converts into monocyclic monoterpenes rather than into the polycyclic monoterpenes that form in He gas or water vapor. A plausible explanation for this is that Lewis acid sites on the surfaces of the catalysts convert to Bronsted acid sites in HCW owing to the stabilization of H+. In HCW, alpha-pinene is converted into acyclic monoterpenes less with the catalysts than without. Furthermore, the selectivities of the TiO2 and the WOx/TiO2 catalysts for monocyclic monoterpenes were quite different, which was attributed to the rapid formation of alpha-terpinene and gamma-terpinene from alpha-pinene and isoterpinolene from terpinolene over the strongly acidic WOx/TiO2 catalyst. No significant coking of the TiO2 and the WOx/TiO2 catalysts occurred in HCW, and both retained their activity for 6 h.