A clean process has been developed for the synthesis of p-menthane-3,8-diols from cyclization of citronellal in CO₂–H₂O medium without any additives. With the addition of CO₂, the reaction rate could be enhanced about 6 times for the cyclization of citronellal in H₂O, because CO₂ dissolved into water and formed carbonic acid inducing an increase of the acidity. Although, the reaction conversion in CO₂–H₂O is slightly lower compared to that obtained with sulfuric acid as catalyst, CO₂–H₂O could replace the sulfuric acid at a relative higher reaction temperature. The reaction kinetics studies showed that the hydration of isopulegols to p-menthane-3,8-diols is a reversible reaction. The equilibrium constant and the maximum equilibrium yield obtained in CO₂–H₂O at a range of CO₂ pressures are similar to that with sulfuric acid catalyst.