The 11B NMR technique has been employed to study the structure of glasses in the system Li2O-B2O3-Al2O3. The values of N4, the fraction of borons in tetrahedral coordination with oxygen, were measured and analysed as a function of R and K (where R=mol% Li2O/mol% B2O3, K=mol%; Al2O3/mol% B2O3). The results indicate that: the formation of four coordinated borons and the formation of four coordinated aluminiums associated with the network modifier lithium oxide are competing processes in the region RR1st, only ALO4 units exist; The R1st glasses for all K families consist mainly of the lithium di-pentaborate units and the first modified lithium diborate units; The Rmax (N4 maximum point) glasses for all K families consist mainly of the lithium diborate units and the second modifified lithium diborate units.
