We carried out the dehydration of propylene glycol in near-critical water to study the reactivity of diol under high temperature and pressure. The end product, 2-methyl-2-pentenal (MP), was identified by GS-MS. The yield of MP was 1.8 wt.% in pure water at 300degreesC for 120 min. As the catalyst, zinc chloride (ZnCl2) and sodium carbonate (Na2CO3) were added in the reaction mixture to study the effect of salts on the dehydration of diol. The yield of MP increased remarkably to reach 59.0 wt.% in 1.00 wt.% ZnCl2 aqueous solution at 300degreesC for 60 min. By contrast, MP was not observed and propylene glycol was hardly converted with the addition of Na2CO3. These demonstrated that ZnCl2 promoted the dehydration of propylene glycol and Na2CO3 suppressed that reaction. The influence of experimental conditions on the reaction was also examined. We proposed the reaction pathway of propylene glycol in near-critical water. The investigation of reaction mechanism indicated that there are two mechanism, pinacol rearrangement and elimination, to be possible in the dehydration of propylene glycol. The impact of ZnCl2 and Na2CO3 on rearrangement and elimination determined their catalytic effects in the whole of reaction. (C) 2003 Elsevier B.V. All rights reserved.