Biosensors capable of directly detecting low levels of formaldehyde and ethanol vapour were constructed. Both biosensors are based on dehydrogenase enzymes which produce reduced nicotinamide adenine dinucleotide as part of the oxidation of formaldehyde and ethanol. The enzymes were immobilized in a reverse micelle medium which did not dehydrate significantly over time, and allowed direct gas-phase monitoring. A screen-printed electrode was used as transducer. Formaldehyde and ethanol vapour partitioned into the reverse micelle media, where it was acted upon by the relevant enzyme. Reduced nicotinamide adenine dinucleotide was oxidized at the working electrode at a potential of 800 mV versus an Ag/AgCl reference electrode. Formaldehyde could be measured over the concentration range 1 ppb–1.3 ppm and ethanol could be detected over the range 50–250 ppm.