ScAlMgO4 is a potential substrate for GaN epitaxy. We have compared three different plasma chemistries for dry patterning of ScAlMgO4, namely Cl2−, F2− or CH4/H2-based. Significant etch rates (>1000Å.min−1) were obtained only with Cl2 (and BCl3), and the rates were directly proportional to both ion energy and ion density in the plasma. Since the etching is ionassisted under all conditions, extremely anisotropic sidewalls are produced on patterned features. Of the wet chemistries investigated at 300K, only HF wet chemical solutions were found to etch ScAlMgO4, although HNO3 can be used at ≤150°C for removal of substrate polishing damage. Hydrogen as 2H has been incorporated into ScAlMgO4 by both ion implantation and by exposure to a plasma at 250°C. In the implanted material diffusion begins at ˜500°C and most of the hydrogen is lost by ≤ 750°C. This thermal stability for hydrogen retention is considerably lower than for other substrate materials for GaN epilayer growth, such as Al2O3 and SiC. There is minimal permeation of 2H from a plasma at 250°C (DH ˜5×10−16 cm2·s−1) in ScAlMgO4, and thus unintentional hydrogen incorporation into GaN overlayers should be minimal at typical growth temperatures.