Diverse crystal phases of bismuth (Bi) oxides induced by the addition of different amounts of tantalum (Ta) were synthesized. Their optoelectronic and redox properties were quantitatively investigated using combined experimental and computational approaches. Synthesis conditions that transform α-Bi2O3 into β-Bi2O3 and δ-Bi2O3 in terms of the Ta quantity, as well as synthesis temperatures, are identified and demonstrated. The phase transition behavior and crystal structures were characterized by in situ high temperature X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG/DTA), X-ray absorption near edge structures (XANES), and extended X-ray absorption fine structures (EXAFS). Density functional theory calculations employing the HSE exchange–correlation functional with spin–orbit coupling were used to quantitatively simulate the optoelectronic properties and band structures of β-Bi2O3 and δ-Bi2O3. Along with the absorption coefficient and density of states, effective masses and dielectric constants were elucidated. The characterization study confirmed the distortion of Ta–O bonds in the Ta-supplemented β-Bi2O3 and the substitutional positions of the Bi and Ta atoms in the δ-Bi2O3 and Bi3TaO7 compounds. The reducibility of these oxides was strongly influenced by the crystal phase confirmed by temperature-programmed reduction (TPR) analysis. These findings can be used as a bismuth oxides' benchmark for optoelectronic applications as well as thermal catalysis as the redox active center or the support.