In this manuscript, Aspirin and 2,4-dinitrochlorobenzene were used as substrates of hydrolysis reactions. The hydrolysis kinetics of the two substrates in microemulsion system for CTAB/n-butanol/25%n-octane/water was studied by using UV method, and the mechanism of the hydrolysis in different microemulsion media, especial the one of aspirin, was discussed. All experimental results showed that the hydrolysis reaction was extremely affected by the structures and the structural transitions of microemulsions. The hydrolysis rates are higher in the water in Oil(W/O) microemulsion media and decrease with the addition of water, in bicontinuous(B.C), they increase with the addition of water, and decrease in the oil in water(O/W). The transition points of the hydrolysis rates occurred just at structural transition points from W/O to B.C. in the microemulsions and from BC. to O/W. The phenomenon was considered to be caused by the different hydrolysis mechanisms in the different microemulsion media and the change of the polarity in “the interfacial films” of microemulsions. The aim of the investigation is to establish what is the connection between the structures of microemulsions and the hydrolysis rates.