The orthorhombic phases LaB3.6O7 and PrB3.6O7 were obtained by high-pressure/high-temperature syntheses at 11 GPa and temperatures of 1130 degree celsius (LaB3.6O7) as well as 850 degree celsius (PrB3.6O7) using a modified Walker-type multianvil apparatus. The title compounds crystallize in the space group Pmn2(1 )(no. 31) exhibiting a defective alpha-SrB4O7 structure variant with the unit cell parameters a=10.4551(3) angstrom, b=4.4562(1) angstrom, c=4.2973(1) angstrom for LaB3.6O7 and a=10.4305(3) angstrom, b=4.4257(1) angstrom, c=4.2815(1) angstrom for PrB3.6O7. Both compounds were characterized by powder/single-crystal X-ray diffraction, vibrational spectroscopy, second harmonic generation, and electron dispersive X-ray spectroscopy measurements.