The domain structure of BiFeO3 was studied by using transmission (4-800K) and reflection (300-1100K) microscopy. The rhombohedral phase transforms at ∼1083 K to a noncubic twinned phase, stable up to the decompn. at 1185 K. Because of a strong neg. temp. shift (∼-5.9 cm-1 K-1) of an absorption edge BiFeO3 turns opaque upon heating. At 15,600 cm-1 the 6A1g → 4T2g crystal field transition of Fe3+ occurs. The maximal spontaneous birefringence of the rhombohedral phase is very high (Δnγ,α = 0.58 at λ = 644 nm and room temp.) and was measured at 10-800K.