β-Ga 2 O 3 , a semiconductor material, has attracted considerable attention given its potential applications in high-power devices, such as high-performance field-effect transistors. For decades, β-Ga 2 O 3 has been processed through chemical mechanical polishing (CMP). Nevertheless, the understanding of the effect of OH − on β-Ga 2 O 3 processed through CMP with an alkaline slurry remains limited. In this study, β-Ga 2 O 3 substrates were successively subjected to mechanical polishing (MP), CMP and etching. Then, to investigate the changes that occurred on the surfaces of the samples, samples were characterised through atomic force microscopy (AFM), three-dimensional laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). LSCM and SEM results showed that β-Ga 2 O 3 is highly vulnerable to brittle fracture during MP. AFM revealed that an ultrasmooth and nondamaged surface with a low R a of approximately 0.18 nm could be obtained through CMP. XPS results indicated that a metamorphic layer, which mainly contains soluble gallium salt (Ga(OH) 4 − ), formed on the β-Ga 2 O 3 surface through a chemical reaction. A dendritic pattern appeared on the surface of β-Ga 2 O 3 after chemical etching. This phenomenon indicated that the chemical reaction on the β-Ga 2 O 3 surface occurred in a nonuniform and selective manner. The results of this study will aid the optimization of slurry preparation and CMP. β-Ga 2 O 3 , a semiconductor material, has attracted considerable attention given its potential applications in high-power devices, such as high-performance field-effect transistors.