A stable electrochemical device for monitoring a component in ambient atmosphere or a gas stream is of the kind which employs a deliquescent electrolyte, is suitable for monitoring and/or analysing hydrogen fluoride, employs a bromide/bromate electrolyte, and employs a cathode which has a large contact area with the electrolyte/atmosphere or gas stream when compared with the anode. The anode can be isolated from direct contact with hydrogen fluoride or with the atmosphere/gas stream containing the former. Control instrumentation and a power pack are preferably compact and portable and can be separate from though connected to the probe head of the device so that the device can be manually employed to monitor any plant leakage of hydrogen fluoride.
