Abstract: A thermal gas flow sensor and method of forming such a sensor. The sensor has a substrate and a heater disposed on the substrate. At least one pair of thermal sensing elements is disposed on the substrate either side of the heater. A protective layer is disposed on at least the heater and/or the thermal sensing elements. The protective layer comprises a high temperature resistant polymer based layer which is preferably a fluoropolymer based layer. The protective layer can also cover interconnects and electrical connections also formed on the substrate so as to completely seal the sensor. A passivation layer, such as silicon nitride, can be disposed on the sensing and/or heating elements and optionally the interconnects and is arranged to interpose the protective layer and the substrate.