A temperature monitoring arrangement for an electro-thermal ice protection system is provided for a slat of an aircraft wing, incorporating heater mats of composite material, wherein, in order to avoid or at least substantially reduce some of the problems associated with use of discrete temperature sensing elements, one or more optical fiber temperature sensors are embedded within the heater mats for sensing an overheating condition of each mat. Advantageously, a control system is used to monitor the temperature of the mats, and adjusts power supplied, or switches off power, in response to detection of an overheating condition.