The glass transition temperatures, T g , of a large number of lithium borosilicate glasses were measured over an exceptionally wide range of alkali oxide to boron oxide ratios, R, for several fixed silica to boron oxide ratios, K. In addition, T g s were measured in the JLi 2 O.SiO 2 system over the range 0 < J <<1.75. In the borosilicates, for each K family two maxima were found in the trends for T g : the first was associated with a maximum in the fraction of four-coordinated borons present; the second was associated with the orthoborate-orthosilicate composition. The trends for T g were analyzed on the basis of a proportional sharing of the alkali with the berate and silicate subsystems for R above a certain critical value of R defined as R 0 . The effects due to carbon dioxide retention were observed in the trends for T g and are discussed. In the silicates, the T g was found to decrease slowly from J = 0.2 through J=1.3. Above J=1.3, the T g increased again until the end of the glass-forming range was reached near J=1.75.