Sulphur hexafluoride (SF6) is used as an insulator in high voltage systems. Electrical breakdown in such a system dissociates SF6 into sulphur fluorides, sulphur and fluorine. Some of these products react with contaminants such as water vapor and oxygen, and with metal surfaces and electrodes, to produce by-products including gaseous sulphur oxyfluorides, hydrogen fluoride and solid by-products. A reduction in SF6 concentration occurs, and the insulating efficiency of the system is degraded. If the SF6 is not renewed, failure of the system can occur. A monitoring system for SF6 loss is required, and since SF6 insulating systems involve high voltages and electromagnetic interference, optical monitoring provides the best approach. This paper describes the development of an optical sensing technique for continuous monitoring of SF6 degradation in high-voltage switchgear. Dissociation products of the SF6 discharge include HF and atomic fluorine, both of which react readily with silica. This reactivity is utilized in our sensor, where etching of a glass fiber surface modifies the transmission characteristics of the fiber. By monitoring the transmitted signal, information on SF6 dissociation is obtained. The sensor is economical to produce, easy to use, and can be readily retrofitted.