Novel redox-active thermosensitive polymers (phenothiazine-labeled poly(ethoxyethyl glycidyl ether), PT-PEEGE), composed of a polyoxyalkylene backbone, ethoxyethoxymethyl side chains, and an electroactive phenothiazine end group, were prepared by base-catalyzed anionic ring-opening polymerization of ethoxyethyl glycidyl ether monomer in the presence of 10- (2-hydroxyethyl)phenothiazine. Phase separation of a 1.0 mmol dm(-3) (0.33 wt %) PT-PEEGE aqueous solution occurs at 28 degreesC. While the phase separation temperature (T-c) is almost constant in the concentration range above 1.0 mmol dm(-3), it increases at below 1.0 mmol dm(-3). A 10-fold decrease in the oxidation current of PT-PEEGE is observed above T-c and reflects the decrease in the apparent concentration of electroactive PT-PEEGE due to the phase separation. The redox response mainly comes from PT-PEEGE molecules in the dilute phase, resulting from the phase separation, and the half-wave potential and peak separation are independent of the phase separation. This thermally induced change in the redox response is reversible and is applied for the thermal control of the electrocatalytic reaction of glucose oxidase (GOx). The catalytic current in the presence of PT-PEEGE as an electron mediator decreases at temperatures higher than T-c. This originates from the phase separation of PT-PEEGE, and PT-PEEGE molecules which remained to be soluble participate in the electrocatalytic reactions of GOx as mediators.