The heats of solution of glasses and devitrified alkali borates in the systems Li2O–B2O3, Na2O—B2O3, and K2O–B2O3were measured in 2 N nitric acid with a simple vacuum‐bottle calorimeter. The standard deviation was 0.35 cal. per gm. for approximately 130 determinations. From these data, together with available thermodynamic data, heats of formation were calculated for alkali borate glasses and devitrified alkali borates from two combinations of possible reactants. All the curves for the heats of solution of glasses showed minima at 20 mole % alkali oxide, but similar curves for devitrified alkali borates showed minima at 20 mole % for potassium and sodium borates only. The curve for devitrified lithium borates showed a minimum at 25 mole % alkali. The conclusion was drawn that there is probably no congruently melting Li2O.4B2O3c