This study details the first application of molecular beam mass spectrometry to investigate the release of sodium, potassium, sulfur, and chlorine during black liquor combustion, Two samples, black liquor with 37.1% dry solids and a sample of dry black liquor solids, were combusted in a quartz-tube reactor at 1100 degrees C in 5% O-2 in helium, The gaseous black liquor combustion products, including alkali metal salts, were monitored continuously and in real time during the combustion event with a molecular beam mass spectrometer (MBMS). Combustion of the dry black liquor solids occurred in multiple phases distinguished as a devolatilization phase and a char combustion phase that was arbitrarily divided into three separate phases, Combustion of the wet black liquor samples occurred in two phases, namely, a combination of water evaporation and devolatilization, and char combustion, NaCl was the most abundant gas-phase alkali-metal-containing species measured during black liquor combustion, The experimental results showed evidence that gas-phase NaOH was also formed during combustion of both black liquor samples. In addition, gas-phase Na2SO4 was observed during combustion of the dry black liquor solids.