The importance of on-line process control in chemical plants is increasing constantly as a result of the high level of automation in production, stricter customer requirements with regard to quality, and considerations of environmental protection. We have developed a fiber optical photometer system for the performance of a wide variety of online measurements. Specific sensors can easily be installed in chemical plants and explosive environments and are linked via glass fibers to an optoelectronic device, which can be more than 100 m away. Our photometers are designed for long performance without any service. Some of more than a hundred applications are discussed.