Lithium niobate (LiNbO 3 ) is a widely used ferroelectric material with high electro-optic, non-linear optic, birefringent and photorefractive activities. Modification of the surface of LiNbO 3 single crystals is essential for its application in optoelectronics. We accomplished the surface modification of single-domain LiNbO 3 wafers (z -cut) in the hydrogen plasma of radiofrequency (r.f.) discharge (13.56 MHz) under pressures of 0.5-3 mbar. Modification of the wafer surface is demonstrated mainly by oxygen and lithium depletion. The overall depth of the modified surface region is ~0.5 µm. There is a thin near-surface layer (<10 nm) where the niobate structure is destroyed and only niobium oxide remains there. Towards the bulk the niobate structure is preserved, although with many point defects. From a depth of 1 µm the genuine niobate composition remains. A double inversion of the spontaneous polarization in the layer was found after reoxidation of the plasma-processed sample. The polarity inversion is adjoined to the internal fields induced by point defect gradients in the plasma-modified surface. The following methods are used: neutron depth profiling for lithium depth distribution; RBS for heavier element depth profiles; x-ray diffraction for evaluation of crystallinity changes; and XPS for composition and chemical bonding of elements in the actual surface.