The feasibility of a diamond-like carbon (DLC) electrode for brake fluid monitoring is presented. The DLC electrode was applied to the determination of water-in-oil concentration in brake fluids by using chronoamperometry and RC-time measurement. From the chronoamperometric method, the DLC electrode could detect water-in-oil concentration of less than 1% at the fixed sampling time (7 ms) after applying a step potential. The RC-time measurement was carried out by using a home-made module which evaluated time for charging current to reach a specified value. The reproducibility of RC-time measurement was evaluated to be less than 5%. (C) 2009 Elsevier B.V. All rights reserved.