Abstract A new biosensor for the determination of the ethanol content of liquids or organic solvents, such as leadless petrols has been developed. The new biosensor is also quite peculiar: it is a, “competitive substrate” organic phase enzyme electrode (OPEE), in which the enzyme, catalase, is, coupled to an amperometric gaseous diffusion Clark type oxygen electrode. The biosensor was, optimized to operate in decane solvent. The new OPEE was applied to the analysis of ethanol in, “leadless petrol”.