The isomerisation of a-pinene oxide to campholenic aldehyde was investigated over a number of solid acids-B2O3/SiO2, ZnCl2/MCM-41, Al-MCM-41, HY zeolites and Al-MSU-S-FAU. Irrespective of the loading, the selectivity to campholenic aldehyde over B2O3/SiO2 was similar to69% at 25 degreesC. The initial rate for the isomerisation of a-pinene oxide was a maximum at 15 wt.% B2O3 loading. Both Lewis and Bronsted acid sites were present in B2O3/SiO2 samples although at higher loadings of B2O3, Bronsted acidity predominated. Under anhydrous conditions, no crystalline phases of boron oxide were detected up to a B2O3 loading of 20 wt.%, but upon exposure to ambient environment, boric acid was formed. The Lewis acid, ZnCl2/MCM-41, was not very active but highly selective (85%) for campholenic aldehyde. Al-MCM-41 and HY zeolites showed high conversion but the selectivity was dependent on the Al content, varying from 49 to 66%. By incorporating both microporosity and mesoporosity in mesostructured Al-MSU-S-FAU (Si/Al 70), good activity and selectivity to campholenic aldehyde, 86%, was observed. (C) 2004 Elsevier B.V. All rights reserved.