This paper describes the design, characterization, and use of an optical bi
osensor suited for the process control of biotechnological processes. The d
etector principle is based on reflectometric interference spectroscopy (RIf
S). RIfS enables a label-free, product-specific monitoring, with a future o
utline for on-line process control. The potential of the RUS biosensor is e
xemplified by the qualitative and quantitative monitoring of the microbial
production of vancomycin-type glycopeptide antibiotics.