A mass-spectrometric Knudsen effusion method has been used for thermochemical study of the vaporization of alkali borosilicate glasses in the R2O-Cs2O-B2O3-SiO2 system, where R = Li, Na, K and Rb. Vapours of RBO2(g), CsBO2(g), R2(BO2)2(g) and RCs(BO2)2(g) have been identified. Over the glasses in which the molar ratio of (R2O + Cs2O)/B2O3 is slightly larger than unity, vapours of R(g), except for Li(g), have been further observed in the initial stage of heating; over the glass containing lithium, however, the vaporization of Cs(g) has been found. Neither R(g) nor Cs(g) has been identified over the glasses in which the molar ratio is equal to unity. Partial pressures of CsBO2(g) over the glasses containing lithium and sodium are lower than those with potassium and rubidium, indicating that chemical activities of the CsBO2 pseudo-component in the glasses with lithium and sodium are small. Enthalpies of formation as well as dissociation energies for dimeric vapours of alkali metaborates are not very different from one another, except that the dissociation energies for the dimeric vapours containing lithium are larger than for those containing the other elements.