The determination of the effective masses in Ga 2 O 3 and In 2 O 3 is rather difficult because of the low crystal symmetry of the α, ε, γ, κ and ω polymorphs, which requires the study of tensors in appropriate coordinates, the nonparabolicity of bands near an extremum and, in the case of hole masses, the determination of the van Hove singularity in k-space as well as the flatness of the valence bands. Moreover, the QP effects strongly influence the electron masses, which is not in line with the application of the questionable scissorsoperator approximation. We found a significant increase in the electron masses within the DFT+A-1 /2 approach compared to the values extracted from DFT as already illustrated in Due to the flat valence bands and the challenges in determining the position of the VBM, which result in very large hole masses and even variations in their sign, we focus on the electron masses for the CBM at the Γ point. To account for their anisotropy, we examine two or even three different directions in the BZ, extending from the Γ point to the BZ boundary.