The degree of proton transfer and thermodynamic parameters of six acetate-based protic ionic liquids (PILs) were measured using nuclear magnetic resonance spectroscopy and calculated by van't Hoff analytical method, respectively. The degree of proton transfer of these PILs at 298 K spread over a large range, which is from 39.6% (1-methylpyrrolidinium acetate, [MpyrH][AcO]) to 94.4% (1-butylimidazo-lium acetate, [BuimH][AcO]). The calculated standard. enthalpy change of the reaction (Delta H-r(m)theta) is from -23.30 to -7.80 kJ mol-(1 )and the standard entropy change of the reaction (Delta H-r(m))(theta) is from -42.70 to -8.07 J mol-K-1-(1) The correlation between the degree of proton transfer and aqueous Delta pK(a) or Delta(r)H(m)(theta)was investigated as well. Furthermore, in some special cases, Delta S-r(m)theta, especially the entropy change of symmetry, also plays an important role in affecting the degree of proton transfer.