III-nitride high electron mobility transistors (HEMTs) are of interest for next generation RF communications owing to their excellent electrical and thermal properties, which should facilitate smaller and more robust systems compared with existing Si and GaAs technologies . For Space applications, there is additional interest in the nitrides due to their superior radiation hardness and chemical stability that maybe required for extreme environments such as the proposed EJSM-Laplace mission to Jupiter [2, 3]. Reliable, low resistance Ohmic contacts are essential in order to maximise the gain and frequency of operation of such HEMTs. Similar to GaN-based photonic devices, the Ti/Al is the most widely employed Ohmic contact scheme on InAlN and AlGaN although higher anneal temperatures are required to realise Ohmic behaviour compared with that of GaN. We have studied the Ti/Al metalisation scheme and high temperature (600°C – 1000°C) anneal requirements for obtaining Ohmic contacts on InAlN and AlGaN HEMT structures. It was found that InAlN requires higher (800°C or greater) anneal temperatures compared to AlGaN to obtain an Ohmic contact and that the choice of diffusion barrier (Ni or Pt) is crucial in determining whether the contact is Ohmic or not.