Bismuth oxide carbonate was synthesized from bismuth nitrate and potassium carbonate and then converted to phase pure β-Bi₂O₃ form by means of thermal decomposition. X-ray diffraction, HR-SEM, diffuse reflectance UV–vis and photocatalytic degradation studies were carried out on both the samples. Bi₂O₂CO₃ exhibited a wide band gap of 3.406(5) eV while β-Bi₂O₃ had a lesser band gap of 2.589(3) eV. β-Bi₂O₃ degrades a higher amount of methyl orange because of its lesser band gap and its optimum loading was 0.1 g in 50 ml of 10 ppm solution. After photocatalytic degradation Bi₂O₂CO₃ remains in the stable form whereas β-Bi₂O₃ changes to Bi₂O₂CO₃.