29Si MAS‐NMR measurements have been made on a series of lithium borosilicate glasses of general compositionRLi2O.B2O3·KSiO2. At low alkali contents (R<1), the29Si resonance envelope is broadened and indicates a distribution of Si sites. AsRincreases above 1, the FWHM of the29Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the11B NMR studies of these same glasses, the29Si NMR data are quantitatively best‐fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins atR= 0. Models of sharing developed from the11B NMR studies of these glasses, where proportional sharing above a certain fixed (independent ofK) or variable (dependent onK) minimumR0, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unk