Wet chemical lift off in N-Methyl-2-pyrrolidone (NMP) is widely used in GaN HEMT Front End manufacturing.  In case of a Ti-Al-Ni-Au based metal stack for ohmic contacts, the quality of the lift-off process is much depending on the water content in the solvent NMP. In this paper, it will be shown that the metal stack can be attacked during lift off in NMP with too high water content. Additionally, environmental impacts on the hygroscopy of NMP are investigated in order to keep moisture below a certain level and avoid optical defects on ohmic contacts after lift off.