The cyclisation of citronellal to isopulegol is an important step in the synthesis of menthol. Several zirconia-based catalysts were evaluated for this reaction. Zirconium hydroxides and phosphated zirconia had very good activity and selectivity. Ammonia TPD and pyridine IR studies indicate the presence of strong Lewis together with weak Bronsted acid sites. Other strong acids like sulfated zirconia, Amberlyst, and Nafion were very active but the selectivity toward isopulegol was poor. They catalysed side reactions such as dehydration, cracking, and etherification of isopulegol. Silica, with only weak Lewis acidity, showed very low activity. The presence of both Lewis and Bronsted sites is therefore essential for the reaction. A reaction mechanism is proposed where the citronellal molecule binds to a zirconium Lewis acid site via the aldehyde oxygen and the jr-electrons of the double bond so that the right configuration is attained for cyclisation. Subsequent protonation of the aldehyde via a neighbouring Bronsted acid site initiates the cyclisation to isopulegol.