Calcium bismuthate nanoparticles with orthorhombic and rhombohedral crystal lattices were synthesized and characterized by various techniques. The syntheses caused the formation of oxygen and cationic vacancies in bismuth-deficient samples. Heat treatment of samples with anionic vacancies led to thermal annealing of oxygen vacancies in ambient air; the annealing process was reversible. Oxygen vacancies in Bi-deficient CaBi₆O₁₀ governed the optical absorption beyond the fundamental absorption edge at E_(hν) < E_bg ≈ 2.5 eV. Bandgap energies of the bismuthates with orthorhombic and rhombohedral lattices were 2.51 eV prior to annealing and 2.47 eV after annealing at 350 °C. The trend in photoactivity toward the degradation of methylene blue dye under visible light showed that the photoactivity of the orthorhombic samples (1-3) increased with a decrease in the [Ca]:[Bi] ratio (1.0:6.3 to 1.0:4.3); the sample with lowest ratio (1.0:4.3) displayed highest photoactivity. By contrast, the rhombohedral samples (4-6) showed increased photoactivity with increase in the number of Bi atoms; sample with highest ratio (1.0:6.3) displayed highest photoactivity.