We investigated the possibility of proton exchange reaction between protic ionic liquid of diethylmethylammonium trifluoromethanesulfonate ([dema][TfOH]) and H2O by using NMR spectroscopy and theoretical calculation. NMR spectroscopy experiment showed that the proton exchange between [dema][TfOH] and H2O in a 1:1 M ratio occurred at room temperature. From the theoretical calculation, we found two reaction pathways for proton exchange between [dema][TfOH] and H2O. Both pathways have stepwise reaction with two transition states. After finishing the reaction along these pathways, the same complex structures of [dema][TfOH] and H2O are again formed, respectively. But the proton is exchanged between [dema][TfOH] and H2O. (C) 2012 Elsevier B.V. All rights reserved.