Living phenol-oxidizing Bacillus stearothermophilus cells were immobilized in a hydroxyethyl methacrylate membrane. A thermostable biosensor was developed by coupling the bacterially activated membrane with a dissolved oxygen electrode and was utilized for the amperometric phenols. Linear relationships were phenol, catechol and some related compounds. The steady-state response was very fast (max. 2 min), faster than other analogous biosensors. Its response was stable, reproducible for months and quite specific. The biosensor may be utilized over a wide temperature (35-55 o C), pH (4.5-8.0) range and in matrices containing compounds toxic for most microorganisms and enzymes used. The best performance was observed at 55 o C and pH 7.2. Owing to its sensitivity, stability and operational simplicity, the phenol biosensor can potentially be applied for the on-line monitoring of phenols in industrial …