The batch reaction experiments have been made for the first time to investigate the effect of ionic structure on the reactivity and selectivity of the trioxane-forming reaction catalyzed by a Brønsted-acidic ionic liquid (IL). The ILs considered include 1-cyclohexyl-2-pyrrolidinonium trifluoromethanesulfonate ([NCyP

]

[TfO

]

)

1-cyclohexyl-2-pyrrolidinonium benzenesulfonate ([NCyP

]

[BSA

]

)

1-cyclohexyl-2-pyrrolidinonium p-toluenesulfonate ([NCyP

]

[p-TSA

]

)

1-octyl-2-pyrrolidinonium 2

4-dinitrobenzenesulfonate ([NOP

]

[DNBSA

]

)

1-octyl-2-pyrrolidinonium benzenesulfonate ([NOP

]

[BSA

]

)

1-octyl-2-pyrrolidinonium methanesulfonate ([NOP

]

[MSA

]

)

and 1-octyl-2-pyrrolidinonium trifluoromethanesulfonate ([NOP

]

[TfO

]

). It is found that the yield of trioxane in the reaction solution correlates inversely with the Hammett acidity function

H

0

of the aqueous solution of the corresponding ILs. Variation of the cation structure from[NCyP

]

+

to[NOP

]

+

exerts little influence on the yield and the selectivity of trioxane in the reaction solution. Using[TfO

]

-

or[DNBSA

]

-

in place of[MSA

]

-

or[BSA

]

-

apparently increases the yield of trioxane

but only

slightly increases the concentration of formic acid in the reaction solution. The continuous production experiments have been made to investigate the performance of[NOP

]

[MSA

]

[NOP

]

[DNBSA

]

[NCyP

]

[TfO

]

and H

2

SO

4

as an extraction distillation agent. Such effect of[NCyP

]

[TfO

]

considerably overrides that of H

2

SO

4

. The yield and the selectivity of trioxane are both increased when[NCyP

]

[TfO

]

is used instead of H

2

SO

4

at a reaction time 5 h.