Abstract Optical basicities (Γ) for Cs 2O + B 2O 3 and Li 2O + B 2O 3 glasses have been measured as a function of glass composition, using Tl +, Pb 2+ and Bi 3+ probe ions. The three probe ions register different values of Γ for glasses of given composition (and also for pure B 2O 3 glass and water). The divergence decreases as the alkali metal ion size decreases. For the Li 2O + B 2O 3 glasses, ideal (calculated) optical basicities agree within experimental precision with experimental values registered by Pb 2+ (Γ Pb 2+) up to about 15 mol% Li 2O. For higher Li 2O contents, and for the Cs 2O + B 2O 3 glasses, ideal optical basicities agree less well with Γ Pb 2+, but show similar trends with composition to those shown by Γ Pb 2+.