To improve the stability of optochemical sensors the immobilisation of dextran labeled with fluorescent dyes in sol-gel films was investigated. The results of a pH-sensor based on dextran labeled with fluorescein were compared to sensors made by physical entrapment of the dye in the sol-gel matrix. The use of fluorescein-dextran for sensor preparation increased stability drastically. To develop stable mercury-sensitive films porphyrin derivatives were covalently bound to dextran and immobilized according to the procedure used for pH sensors. The stability of the resulting mercury sensors was longer than 6 weeks.