The selective oxidation of cyclopentene by aqueous H

2

O

2

using H

3

PW

12

O

40

and tetrabutyl ammonium bromide (TBAB) as a phase transfer catalyst has been investigated. The results show that the presence of TBAB significantly improved the oxidation selectivity of cyclopentene. The effects of the reaction conditions on the conversion of cyclopentene were investigated in detail. The optimal reaction conditions are as follows:the H

3

PW

12

O

40

to TBAB molar ratio

1:1-1:3; H

3

PW

12

O

40

to cyclopentene molar ratio

0.54:100-0.64:100; and molar ratio of H

2

O

2

to cyclopentene

1.6:1. The conversion reached to 59.8% in 4 h at 35.0℃

while the selectivity of glutaraldehyde was 38.0% and the selectivity of 1

2-cyclopentanediol was 55.6%. In addition

a route for oxidation of cyclopentene by aqueous H

2

O

2

using a heteropoly acid and quaternary ammonium salt as a phase transfer catalyst was proposed.