Fluoride plasma treatment similar to that used in AlGaN/GaN HEMTs has been applied to InAlN/GaN HEMTs. Enhancement mode of operation is obtained with a pinch-off voltage shifted by 3 V. Owing to the fluoride treatment an increase of the forward gate threshold to 3.5 V is observed. The small-signal performances are essentially unchanged. The thermal stability of this process has been assessed for the first time and appears to be limited to approximately 500 degrees C.