The design of a compact and reliable surface plasmon resonance (SPR) sensor is described and its use in industrial process control is reported. Its application for probable alcoholic degree (PAD) determination in must samples, integrated in a flow-injection system, is described. The sensor response is first evaluated and optimized under dynamic conditions in order to match its working range to that shown by must samples. The performance of the overall system has been tested during harvest time at a wine production plant. Results are compared with those obtained using the conventional refractometric procedure and show adequate agreement. © 1993.