A flow injection analysis system for on-line monitoring of enzyme-catalyzed L-ornithine, L-methionine, and fructose synthesis is presented. An enzyme thermistor was used as detector. The enzyme-catalyzed reactions were performed in a simple batch reactor with immobilized biocatalysts (arginase, acylase, invertase, or glucose isomerase, respectively). The bioreactor was connected with a computer-controlled FIA system. The method allows continuous monitoring and optimization of an enzyme-catalyzed process.