Boron-modified mesoporous materials MCM-41 have been synthesized and characterized by nuclear magnetic resonance (NMR) spectroscopy. B-11 MAS NMR data show that all the boron in the as-synthesized samples is present in the four-coordinated state even in samples with Si/B ratios as low as 6.25. Results of Si-29 MAS NMR suggest that attachment of boron to the lattice takes place through interaction with structural hydroxyl groups. After calcination, the boron is present mainly in a trigonal coordination state, and only a small amount remained tetrahedrally coordinated. It was found that part of the boron is weakly connected to the lattice and can be easily removed by hydrolysis in moist air at room temperature. The process of hydrolysis was monitored by B-11 MAS NMR. The remaining part of the boron is strongly bound to the lattice and can withstand severe treatments.