The isomerization of α-pinene in the vapor phase over alumina catalysts of varying acid strength has been studied. The isomerization proceeds via two parallel paths, one giving bi- and tricyclic products such as camphene, β-pinene, tricyclene, and bornylene and the other giving rise to monocyclic compounds such as dipentene, terpinolene, α-terpinene, γ-terpinene,p-cymene, andp-menthene. Dependence of the distribution of reaction products on space time reveals that camphene, β-pinene, tricyclene, dipentene, and terpinolene are the primary products. The acid strength of alumina catalysts has a remarkable influence on the selectivity ratio of bi- and tricyclic to monocyclic products, the strong acid sites favoring the formation of monocyclic products. From the study of the reactions of camphene and tricyclene over the same catalysts under similar conditions it has been concluded that the decrease in the selectivity ratio with increasing acid strength of the catalysts is mainly due to the further isomerization of the bi- and tricyclic compounds to monocyclic products which takes place only on the strong acid sites.