The acidic strength of a liquid acid catalyst,a measure of its ability to either donate protons or accept electron pairs,is normally characterized by conventional acid-base titration method under aqueous solution[1],leading to a scaled pKa value.On the other hand,the acidic strengths of non-aqueous and solid acids are commonly classified by amine titration method using Hammett indicators,which may be quantitatively expressed by the Hammett acidity function (Ho) [2].In the past few decades,many attempts have been adopted for the analyses of acidic strengths using calorimetric measurements[3],infrared[4],and NMR[5] spectroscopy,and thermal desorption of adsorbed probe molecules,or application of indicator dyes[6].Previously,we have demonstrated that detailed acid features,viz.types (Br?nsted vs.Lewis),distributions (location and concentration),and strengths,of various solid acid catalysts can be simultaneously obtained by solid-state 31P MAS NMR of adsorbed trialkylphosphine oxides (R3PO)[7].We have also conducted theoretical quantum chemical DFT calculations to interpret the observed 31P NMR chemical shifts (CSs) in various solid acid catalysts[8].