Structure of active site and redox reaction of azurin in a hydrated ionic liquid, cholinium dihydrogen phosphate ([ch][dhp]), was maintained after dissolution of azurin in hydrated [ch][dhp]. Cyclic voltammograms for azurin immobilized on a self-assembled monolayer (SAM) constructed on gold nanoparticle (AuNP) electrodes showed a reversible and stable redox response in hydrated [ch][dhp]. The electron transfer rate (k(s)) between azurin and the electrode in hydrated [ch][dhp] was 202 s(-1), which is larger than that in 10 mM ammonium acetate buffer (k(s) = 44 s(-1)). (C) 2016 The Electrochemical Society. All rights reserved.