Abstract: Transformation of the probe molecular ethanol over the selected isomerization catalyst supports, which were prepared from Alumina, ZSM-5 and SAPO-11 powders, was investigated at different reaction temperatures. Combined with the results of acidity data by NH3-TPD, and the results of longchain C14~24 hydrocarbon hydroisomerization, the relationships among the acidity, ethanol conversion as well as the hydroisomerization performance were investigated. It was shown that neither the ethanol conversion nor the hydroisomerization could correlate with acidity. However, a good relationship between ethanol conversion and hydroisomerization was observed. When a support presented low ethylene selectivity and inferior stability during ethanol conversion, the corresponding hydroisomerization catalyst had bad performance. While a support shown stable activity and high ethylene selectivity for ethanol conversion, a hydroisomerization catalyst with better stability and high isomerization selectivity might be obtained after loading precious metal onto the support. After detailed analysis of these data, we found that it was necessary to discriminate between acidity and acidic property. Acidity includes the acid amount, acid distribution as well as acidic strength, while acidic property, temporally named as acid function, is the acidic functional behavior during a reaction. The former can be characterized by traditional method, but the later can only be probed by a catalytic transformation. The obtained data strongly suggests that it is possible to characterize the acid function of hydroisomerization catalyst by a model reaction of ethanol conversion. The model reaction using ethanol as probe molecule could be used to characterize the acid function of a support, and thus provide useful instructions in selecting hydroisomerization catalyst supports.