Single crystals of iron(II) pyroborate, Fe 2 B 2 O 5 , were prepared at 1000–1050°C under an argon atmosphere. The crystals were transparent, yellowish in color and needle-like or columnar. The crystal structure of Fe 2 B 2 O 5 was analyzed by single-crystal X-ray diffraction. Refined triclinic unit cell parameters were a=3.2388(2), b=6.1684(5), c=9.3866(8)Å, α=104.613(3)°, β=90.799(2)° and γ=91.731(2)°. The final reliability factors of refinement were R1=0.020 and wR2=0.059 [I > 2σ(I)]. Transmittance over 50% in the visible light region from 500 to 750nm was observed for a single crystal of Fe 2 B 2 O 5 with a thickness of about 0.3mm. The light absorption edge estimated from a diffuse reflectance spectrum was at around 350nm (3.6eV). Magnetic susceptibility was measured for single crystals at 4–300K. Fe 2 B 2 O 5 showed antiferromagnetic behavior below the Néel temperature, T N ≈70K, and the Weiss temperature was T W =36K. The effective magnetic moment of Fe was 5.3μ B .