An enzyme biosensor specific for formaldehyde has been developed using pH-sensitive field effect transistors (FET) as a transducer and alcohol oxidase (AOX) from Hansenula polymorpha as a sensitive element. The steady-state response time of the developed biosensor was 10-60 s. The linear dynamic range (in semi-logarithmic scale) of the sensor output signal corresponded to 10-300 mmol dm(-3) formaldehyde concentration. The operational stability of the biosensor was not less than 7 h and the relative standard deviation approximately 1-3%. When stored at +4 degrees C, the response was stable for more than 30 days. The biosensor demonstrated high specificity to formaldehyde with no potentiometric response to primary alcohols. The influence of buffer concentration and enzyme concentration in the biomembrane on the sensor signal was investigated. The specific biosensor could also be successfully used for highly selective, quantitative estimation of mercury ion in an aqueous solution.