A modular optical sensor system for fluid media has a measuring module which includes an exchangeable fluid chamber and an exchangeable optic holder. The fluid chamber has an inlet and an outlet as well as a measurement space for the fluid medium. The optic holder has at least one optical transmitter and at least one optical receiver. The optic holder is inserted within the measuring module relative to the fluid chamber in such a way that the radiation emitted by the optical transmitter traverses the measurement space for the fluid medium in the fluid chamber and impinges on the optical receiver.