Abstract Industrial development of glass-forming processes for nuclear waste disposal, particularly borosilicate glasses, requires basic data on glass vaporization thermodynamics. Using special high-temperature mass-spectrometric methods, species partial pressure data have been obtained for a non-radioactive borosilicate process glass containing simulated nuclear waste isotopes. Alkali metaborates were observed to be dominant vapor species and their partial pressures indicate significant transport under likely process conditions. The relative order of significanc of vapor transport of radionuclides was found to be Cs ≳ Re (∼ Tc) > Ru ⪢ Sr. Significant losses by vaporization can also occur during the initial glass-forming process. Decomposition and hydrolysis of the cesium formate starting material appear to be the significant reactions. Detailed thermal decomposition studies indicate that the degradation of cesium formate to yield cesium carbonate involves a complex stepwise pathway. The resultant carbonate releases Cs(g) at a rate up to an order of magnitude higher than for the pure carbonate due to reduction interactions with the carbon produced by formate decomposition.