Boron suboxide exhibits ultrahigh hardness but poor fracture toughness, which limits its use for extreme applications that require tough materials. One strategy to improve bulk fracture toughness of boron suboxide is to incorporate intergranular films to weaken grain boundaries and promote crack deflection. This study aimed to engineer nanolayer films in Yb-doped boron suboxide and evaluate their potential effect on bulk toughness. As-received boron suboxide powders were co-doped with 5 vol% of silica and ~3 vol% ytterbium oxide additives and hot-pressed at 1850°C for 3 hours at ~54 MPa. Abnormal grain growth was observed which suggested that multiple complexion types existed in the microstructure. Subsequently, differences between “abnormal” and “normal” grain boundary structures were determined using aberration-corrected high-angle annular dark field imaging, differences in compositions were determined using ζ-factor microanalysis, and differences in toughness using site-specific micro-cantilever beam testing. Overall, it was determined that, as compared to the “normal” grain boundaries, the abnormal interfaces were indeed more disordered with Yb and Si absorbates and also exhibited lower fracture toughnesses. Experimental procedures, results, and analyses are presented.