α-Pinene oxide (2) is a very reactive substrate which isomerizes rapidly under the influence of acid catalysts. A large number of products can be formed from a single intermediate. Selectivity to the industrially important campholenic aldehyde can be achieved with metal-halogen compounds, especially zinc chloride and zinc bromide. A possible explanation for this behaviour is offered, stipulating a stabilized carbenium-ion intermediate. Protonic catalysts are typically highly active but unselective. No evidence was found for the earlier reported selectivity to campholenic aldehyde with zeolite catalysts.