A comparative study of urease and penicillinase biosensors was performed in which different transducers were compared, i.e. pH glass electrodes, macroscopic ion-selective electrodes and thick-film transducers. For enzyme immobilization onto the transducers aqueous dispersion polymers were used in all cases in this work. Since these sensors are based on potentiometric measurements the lack of a classical reference electrode when solid state transducers are employed is a crucial feature. Results obtained in this work revealed that thick-film sensors without a classical reference electrode can compensate this deficit when they operate in a differential mode. This means that it is not the absolute potential change of a single selective electrode, induced by the analyte concentration, that is measured but rather the relative change with reference to a non-selective electrode. Enzyme sensors with macroscopic transducers and classical reference electrodes showed the same basic characteristics compared with thick-film sensors. The long term stability of the sensors is sufficient; urease and penicillinase thick-film sensors showed 50 and 75% of the initial response after two weeks of permanent use at room temperature.